diff --git a/Directory.Build.props b/Directory.Build.props
index 34da8f49..1bfa061d 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -32,7 +32,7 @@
     <WarningsNotAsErrors>$(WarningsNotAsErrors);1591</WarningsNotAsErrors>
     <LangVersion>9.0</LangVersion>
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
-    <Nullable>annotations</Nullable>
+    <Nullable>enable</Nullable>
     <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)src\StrongName.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
 
diff --git a/src/CommandLineUtils/Attributes/AllowedValuesAttribute.cs b/src/CommandLineUtils/Attributes/AllowedValuesAttribute.cs
index 3312c1ce..5bdc527b 100644
--- a/src/CommandLineUtils/Attributes/AllowedValuesAttribute.cs
+++ b/src/CommandLineUtils/Attributes/AllowedValuesAttribute.cs
@@ -66,14 +66,14 @@ or StringComparison.InvariantCultureIgnoreCase
         }
 
         /// <inheritdoc />
-        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        protected override ValidationResult? IsValid(object? value, ValidationContext validationContext)
         {
             if (value is string str && _allowedValues.Any(t => str.Equals(t, Comparer)))
             {
                 return ValidationResult.Success;
             }
 
-            return new ValidationResult(FormatErrorMessage(value as string));
+            return new ValidationResult(FormatErrorMessage(value?.ToString() ?? string.Empty));
         }
     }
 }
diff --git a/src/CommandLineUtils/Attributes/FilePathExistsAttributeBase.cs b/src/CommandLineUtils/Attributes/FilePathExistsAttributeBase.cs
index 90bfbb0e..891a7ecc 100644
--- a/src/CommandLineUtils/Attributes/FilePathExistsAttributeBase.cs
+++ b/src/CommandLineUtils/Attributes/FilePathExistsAttributeBase.cs
@@ -27,11 +27,11 @@ internal FilePathExistsAttributeBase(FilePathType filePathType)
         }
 
         /// <inheritdoc />
-        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        protected override ValidationResult? IsValid(object? value, ValidationContext validationContext)
         {
             if (value is not string path || path.Length == 0 || path.IndexOfAny(Path.GetInvalidPathChars()) >= 0)
             {
-                return new ValidationResult(FormatErrorMessage(value as string));
+                return new ValidationResult(FormatErrorMessage(value?.ToString() ?? string.Empty));
             }
 
             if (!Path.IsPathRooted(path)
@@ -50,7 +50,7 @@ protected override ValidationResult IsValid(object value, ValidationContext vali
                 return ValidationResult.Success;
             }
 
-            return new ValidationResult(FormatErrorMessage(value as string));
+            return new ValidationResult(FormatErrorMessage(value?.ToString() ?? string.Empty));
         }
 
         private static string GetDefaultErrorMessage(FilePathType filePathType)
diff --git a/src/CommandLineUtils/Attributes/FilePathNotExistsAttributeBase.cs b/src/CommandLineUtils/Attributes/FilePathNotExistsAttributeBase.cs
index 0b1a1e78..535637ab 100644
--- a/src/CommandLineUtils/Attributes/FilePathNotExistsAttributeBase.cs
+++ b/src/CommandLineUtils/Attributes/FilePathNotExistsAttributeBase.cs
@@ -27,11 +27,11 @@ internal FilePathNotExistsAttributeBase(FilePathType filePathType)
         }
 
         /// <inheritdoc />
-        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        protected override ValidationResult? IsValid(object? value, ValidationContext validationContext)
         {
             if (value is not string path || path.Length == 0 || path.IndexOfAny(Path.GetInvalidPathChars()) >= 0)
             {
-                return new ValidationResult(FormatErrorMessage(value as string));
+                return new ValidationResult(FormatErrorMessage(value?.ToString() ?? string.Empty));
             }
 
             if (!Path.IsPathRooted(path)
@@ -55,7 +55,7 @@ protected override ValidationResult IsValid(object value, ValidationContext vali
                 return ValidationResult.Success;
             }
 
-            return new ValidationResult(FormatErrorMessage(value as string));
+            return new ValidationResult(FormatErrorMessage(value?.ToString() ?? string.Empty));
         }
 
         private static string GetDefaultErrorMessage(FilePathType filePathType)
diff --git a/src/CommandLineUtils/Attributes/LegalFilePathAttribute.cs b/src/CommandLineUtils/Attributes/LegalFilePathAttribute.cs
index 2fc79fd0..45df3ec5 100644
--- a/src/CommandLineUtils/Attributes/LegalFilePathAttribute.cs
+++ b/src/CommandLineUtils/Attributes/LegalFilePathAttribute.cs
@@ -23,7 +23,7 @@ public LegalFilePathAttribute()
         }
 
         /// <inheritdoc />
-        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        protected override ValidationResult? IsValid(object? value, ValidationContext validationContext)
         {
             if (value is string path)
             {
@@ -37,7 +37,7 @@ protected override ValidationResult IsValid(object value, ValidationContext vali
                 }
             }
 
-            return new ValidationResult(FormatErrorMessage(value as string));
+            return new ValidationResult(FormatErrorMessage(value?.ToString() ?? string.Empty));
         }
     }
 }
diff --git a/src/CommandLineUtils/CommandArgument.cs b/src/CommandLineUtils/CommandArgument.cs
index e0330b38..f9c183bf 100644
--- a/src/CommandLineUtils/CommandArgument.cs
+++ b/src/CommandLineUtils/CommandArgument.cs
@@ -78,7 +78,7 @@ public IReadOnlyList<string?> Values
         /// <summary>
         /// Defines the underlying type of the argument for the help-text-generator
         /// </summary>
-        internal Type UnderlyingType { get; set; }
+        internal Type? UnderlyingType { get; set; }
 
         /// <summary>
         /// Try to add a value to this argument.
diff --git a/src/CommandLineUtils/CommandArgument{T}.cs b/src/CommandLineUtils/CommandArgument{T}.cs
index 2bdf7d41..e3492011 100644
--- a/src/CommandLineUtils/CommandArgument{T}.cs
+++ b/src/CommandLineUtils/CommandArgument{T}.cs
@@ -52,9 +52,9 @@ public IReadOnlyList<T> ParsedValues
                     ((IInternalCommandParamOfT)this).Parse(CultureInfo.CurrentCulture);
                 }
 
-                if (_parsedValues.Count == 0 && _hasDefaultValue)
+                if (_parsedValues.Count == 0 && _hasDefaultValue && DefaultValue != null)
                 {
-                    return new[] { DefaultValue };
+                    return new T[] { DefaultValue };
                 }
 
                 return _parsedValues;
@@ -85,7 +85,7 @@ void IInternalCommandParamOfT.Parse(CultureInfo culture)
             }
         }
 
-        void SetBaseDefaultValue(T value)
+        void SetBaseDefaultValue(T? value)
         {
             if (!ReflectionHelper.IsSpecialValueTupleType(typeof(T), out _))
             {
diff --git a/src/CommandLineUtils/CommandLineApplication.Validation.cs b/src/CommandLineUtils/CommandLineApplication.Validation.cs
index a46091b3..a77a5b29 100644
--- a/src/CommandLineUtils/CommandLineApplication.Validation.cs
+++ b/src/CommandLineUtils/CommandLineApplication.Validation.cs
@@ -32,7 +32,7 @@ public Func<ValidationResult, int> ValidationErrorHandler
         /// Validates arguments and options.
         /// </summary>
         /// <returns>The first validation result that is not <see cref="ValidationResult.Success"/> if there is an error.</returns>
-        public ValidationResult GetValidationResult()
+        public ValidationResult? GetValidationResult()
         {
             if (Parent != null)
             {
diff --git a/src/CommandLineUtils/CommandLineApplication.cs b/src/CommandLineUtils/CommandLineApplication.cs
index e5774009..7987e8a0 100644
--- a/src/CommandLineUtils/CommandLineApplication.cs
+++ b/src/CommandLineUtils/CommandLineApplication.cs
@@ -863,7 +863,7 @@ public async Task<int> ExecuteAsync(string[] args, CancellationToken cancellatio
             }
 
             var validationResult = command.GetValidationResult();
-            if (validationResult != ValidationResult.Success)
+            if (validationResult != null)
             {
                 return command.ValidationErrorHandler(validationResult);
             }
diff --git a/src/CommandLineUtils/CommandLineApplication{T}.cs b/src/CommandLineUtils/CommandLineApplication{T}.cs
index 26e4dbc6..6ea6ab53 100644
--- a/src/CommandLineUtils/CommandLineApplication{T}.cs
+++ b/src/CommandLineUtils/CommandLineApplication{T}.cs
@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using McMaster.Extensions.CommandLineUtils.Abstractions;
 using McMaster.Extensions.CommandLineUtils.Conventions;
 using McMaster.Extensions.CommandLineUtils.HelpText;
@@ -15,6 +16,7 @@ namespace McMaster.Extensions.CommandLineUtils
     public class CommandLineApplication<TModel> : CommandLineApplication, IModelAccessor
         where TModel : class
     {
+        [AllowNull] // this is initialized in common method called in all constructors
         private Lazy<TModel> _lazy;
         private Func<TModel> _modelFactory = DefaultModelFactory;
 
diff --git a/src/CommandLineUtils/CommandOption.cs b/src/CommandLineUtils/CommandOption.cs
index ae6a5fb9..f8d7ddee 100644
--- a/src/CommandLineUtils/CommandOption.cs
+++ b/src/CommandLineUtils/CommandOption.cs
@@ -137,7 +137,7 @@ public IReadOnlyList<string?> Values
         /// <summary>
         /// Defines the underlying type of the option for the help-text-generator
         /// </summary>
-        internal Type UnderlyingType { get; set; }
+        internal Type? UnderlyingType { get; set; }
 
         /// <summary>
         /// A collection of validators that execute before invoking <see cref="CommandLineApplication.OnExecute(Func{int})"/>.
diff --git a/src/CommandLineUtils/CommandOption{T}.cs b/src/CommandLineUtils/CommandOption{T}.cs
index 84d18a36..b15ec78d 100644
--- a/src/CommandLineUtils/CommandOption{T}.cs
+++ b/src/CommandLineUtils/CommandOption{T}.cs
@@ -53,7 +53,7 @@ public IReadOnlyList<T> ParsedValues
                     ((IInternalCommandParamOfT)this).Parse(CultureInfo.CurrentCulture);
                 }
 
-                if (_parsedValues.Count == 0 && _hasDefaultValue)
+                if (_parsedValues.Count == 0 && _hasDefaultValue && DefaultValue != null)
                 {
                     return new[] { DefaultValue };
                 }
@@ -86,7 +86,7 @@ void IInternalCommandParamOfT.Parse(CultureInfo culture)
             }
         }
 
-        private void SetBaseDefaultValue(T value)
+        private void SetBaseDefaultValue(T? value)
         {
             if (!ReflectionHelper.IsSpecialValueTupleType(typeof(T), out _))
             {
diff --git a/src/CommandLineUtils/Conventions/ExecuteMethodConvention.cs b/src/CommandLineUtils/Conventions/ExecuteMethodConvention.cs
index 473b15c9..f682bd89 100644
--- a/src/CommandLineUtils/Conventions/ExecuteMethodConvention.cs
+++ b/src/CommandLineUtils/Conventions/ExecuteMethodConvention.cs
@@ -35,8 +35,8 @@ private async Task<int> OnExecute(ConventionContext context, CancellationToken c
             MethodInfo? asyncMethod;
             try
             {
-                method = context.ModelType.GetMethod("OnExecute", binding);
-                asyncMethod = context.ModelType.GetMethod("OnExecuteAsync", binding);
+                method = context.ModelType?.GetMethod("OnExecute", binding);
+                asyncMethod = context.ModelType?.GetMethod("OnExecuteAsync", binding);
             }
             catch (AmbiguousMatchException ex)
             {
@@ -75,7 +75,7 @@ private async Task<int> OnExecute(ConventionContext context, CancellationToken c
             throw new InvalidOperationException(Strings.InvalidOnExecuteReturnType(method.Name));
         }
 
-        private async Task<int> InvokeAsync(MethodInfo method, object instance, object[] arguments)
+        private async Task<int> InvokeAsync(MethodInfo method, object instance, object?[] arguments)
         {
             try
             {
@@ -95,7 +95,7 @@ private async Task<int> InvokeAsync(MethodInfo method, object instance, object[]
             return 0;
         }
 
-        private int Invoke(MethodInfo method, object instance, object[] arguments)
+        private int Invoke(MethodInfo method, object instance, object?[] arguments)
         {
             try
             {
diff --git a/src/CommandLineUtils/Conventions/SubcommandAttributeConvention.cs b/src/CommandLineUtils/Conventions/SubcommandAttributeConvention.cs
index 637127f0..6997b2a0 100644
--- a/src/CommandLineUtils/Conventions/SubcommandAttributeConvention.cs
+++ b/src/CommandLineUtils/Conventions/SubcommandAttributeConvention.cs
@@ -67,7 +67,7 @@ private static readonly MethodInfo s_addSubcommandMethod
         private void AddSubcommandImpl<TSubCommand>(ConventionContext context)
             where TSubCommand : class
         {
-            context.Application.Command<TSubCommand>(null, null);
+            context.Application.Command<TSubCommand>(null!, null!); // Hmm, should probably rethink this...
         }
     }
 }
diff --git a/src/CommandLineUtils/HelpText/DefaultHelpTextGenerator.cs b/src/CommandLineUtils/HelpText/DefaultHelpTextGenerator.cs
index fe5a57ea..1875d3d9 100644
--- a/src/CommandLineUtils/HelpText/DefaultHelpTextGenerator.cs
+++ b/src/CommandLineUtils/HelpText/DefaultHelpTextGenerator.cs
@@ -409,7 +409,7 @@ protected virtual string Format(CommandOption option)
             }
         }
 
-        private string[] ExtractNamesFromEnum(Type type)
+        private string[] ExtractNamesFromEnum(Type? type)
         {
             if (type == null)
             {
diff --git a/src/CommandLineUtils/Internal/CommandLineProcessor.cs b/src/CommandLineUtils/Internal/CommandLineProcessor.cs
index 5ab7d709..5a515784 100644
--- a/src/CommandLineUtils/Internal/CommandLineProcessor.cs
+++ b/src/CommandLineUtils/Internal/CommandLineProcessor.cs
@@ -335,7 +335,10 @@ private bool ProcessUnexpectedArg(string argTypeName, string? argValue = null)
 
                 case UnrecognizedArgumentHandling.CollectAndContinue:
                     var arg = _enumerator.Current;
-                    _currentCommand._remainingArguments.Add(arg.Raw);
+                    if (arg != null)
+                    {
+                        _currentCommand._remainingArguments.Add(arg.Raw);
+                    }
                     return true;
 
                 case UnrecognizedArgumentHandling.StopParsingAndCollect:
diff --git a/src/CommandLineUtils/Internal/ReflectionHelper.cs b/src/CommandLineUtils/Internal/ReflectionHelper.cs
index e22c795d..3a3b7f04 100644
--- a/src/CommandLineUtils/Internal/ReflectionHelper.cs
+++ b/src/CommandLineUtils/Internal/ReflectionHelper.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
 using System.Threading;
@@ -78,10 +79,10 @@ public static MemberInfo[] GetMembers(Type type)
             return GetAllMembers(type).ToArray();
         }
 
-        public static object[] BindParameters(MethodInfo method, CommandLineApplication command, CancellationToken cancellationToken)
+        public static object?[] BindParameters(MethodInfo method, CommandLineApplication command, CancellationToken cancellationToken)
         {
             var methodParams = method.GetParameters();
-            var arguments = new object[methodParams.Length];
+            var arguments = new object?[methodParams.Length];
 
             for (var i = 0; i < methodParams.Length; i++)
             {
@@ -117,7 +118,7 @@ public static object[] BindParameters(MethodInfo method, CommandLineApplication
             return arguments;
         }
 
-        public static bool IsNullableType(Type type, out Type? wrappedType)
+        public static bool IsNullableType(Type type, [NotNullWhen(true)] out Type? wrappedType)
         {
             var result = type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>);
             wrappedType = result ? type.GetGenericArguments().First() : null;
@@ -125,7 +126,7 @@ public static bool IsNullableType(Type type, out Type? wrappedType)
             return result;
         }
 
-        public static bool IsSpecialValueTupleType(Type type, out Type? wrappedType)
+        public static bool IsSpecialValueTupleType(Type type, [NotNullWhen(true)] out Type? wrappedType)
         {
             var result = type.IsGenericType &&
                          type.GetGenericTypeDefinition() == typeof(ValueTuple<,>) &&
@@ -135,7 +136,7 @@ public static bool IsSpecialValueTupleType(Type type, out Type? wrappedType)
             return result;
         }
 
-        public static bool IsSpecialTupleType(Type type, out Type? wrappedType)
+        public static bool IsSpecialTupleType(Type type, [NotNullWhen(true)] out Type? wrappedType)
         {
             var result = type.IsGenericType &&
                          type.GetGenericTypeDefinition() == typeof(Tuple<,>) &&
diff --git a/src/CommandLineUtils/Internal/ValueParsers/TypeDescriptorValueParserFactory.cs b/src/CommandLineUtils/Internal/ValueParsers/TypeDescriptorValueParserFactory.cs
index 34f617ff..46ead25f 100644
--- a/src/CommandLineUtils/Internal/ValueParsers/TypeDescriptorValueParserFactory.cs
+++ b/src/CommandLineUtils/Internal/ValueParsers/TypeDescriptorValueParserFactory.cs
@@ -3,6 +3,7 @@
 
 using System;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 
 namespace McMaster.Extensions.CommandLineUtils.Abstractions
@@ -13,7 +14,7 @@ namespace McMaster.Extensions.CommandLineUtils.Abstractions
     /// </summary>
     internal class TypeDescriptorValueParserFactory
     {
-        public bool TryGetParser<T>(out IValueParser parser)
+        public bool TryGetParser<T>([NotNullWhen(true)] out IValueParser? parser)
         {
             var targetType = typeof(T);
             var converter = TypeDescriptor.GetConverter(targetType);
@@ -39,7 +40,7 @@ public TypeConverterValueParser(Type targetType, TypeConverter typeConverter)
 
             private TypeConverter TypeConverter { get; }
 
-            public T Parse(string argName, string value, CultureInfo culture)
+            public T Parse(string? argName, string? value, CultureInfo culture)
             {
                 try
                 {
@@ -52,7 +53,7 @@ public T Parse(string argName, string value, CultureInfo culture)
                 }
             }
 
-            object IValueParser.Parse(string argName, string value, CultureInfo culture)
+            object? IValueParser.Parse(string? argName, string? value, CultureInfo culture)
                 => Parse(argName, value, culture);
         }
     }
diff --git a/src/CommandLineUtils/Properties/NullabilityHelpers.cs b/src/CommandLineUtils/Properties/NullabilityHelpers.cs
new file mode 100644
index 00000000..7056bc25
--- /dev/null
+++ b/src/CommandLineUtils/Properties/NullabilityHelpers.cs
@@ -0,0 +1,21 @@
+// Copyright (c) Nate McMaster.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+// Files here are for simplify annotations of nullable code and are not functional in .NET Standard 2.0
+#if NETSTANDARD2_0 || NET45
+namespace System.Diagnostics.CodeAnalysis
+{
+    // https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.notnullwhenattribute?
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        public NotNullWhenAttribute(bool returnValue)
+        {
+        }
+    }
+
+    // https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.allownullattribute
+    [System.AttributeUsage(System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.Property, Inherited=false)]
+    internal sealed class AllowNullAttribute : Attribute { }
+}
+#endif
diff --git a/src/CommandLineUtils/PublicAPI.Shipped.txt b/src/CommandLineUtils/PublicAPI.Shipped.txt
index 1476d1c4..b72b2bdf 100644
--- a/src/CommandLineUtils/PublicAPI.Shipped.txt
+++ b/src/CommandLineUtils/PublicAPI.Shipped.txt
@@ -127,7 +127,7 @@ McMaster.Extensions.CommandLineUtils.CommandLineApplication.ExtendedHelpText.set
 McMaster.Extensions.CommandLineUtils.CommandLineApplication.FullName.get -> string?
 McMaster.Extensions.CommandLineUtils.CommandLineApplication.FullName.set -> void
 McMaster.Extensions.CommandLineUtils.CommandLineApplication.GetOptions() -> System.Collections.Generic.IEnumerable<McMaster.Extensions.CommandLineUtils.CommandOption!>!
-McMaster.Extensions.CommandLineUtils.CommandLineApplication.GetValidationResult() -> System.ComponentModel.DataAnnotations.ValidationResult!
+McMaster.Extensions.CommandLineUtils.CommandLineApplication.GetValidationResult() -> System.ComponentModel.DataAnnotations.ValidationResult?
 McMaster.Extensions.CommandLineUtils.CommandLineApplication.HelpOption(string! template, bool inherited) -> McMaster.Extensions.CommandLineUtils.CommandOption!
 McMaster.Extensions.CommandLineUtils.CommandLineApplication.HelpOption(string! template) -> McMaster.Extensions.CommandLineUtils.CommandOption!
 McMaster.Extensions.CommandLineUtils.CommandLineApplication.HelpTextGenerator.get -> McMaster.Extensions.CommandLineUtils.HelpText.IHelpTextGenerator!
@@ -435,9 +435,9 @@ McMaster.Extensions.CommandLineUtils.ValidateMethodConvention.Apply(McMaster.Ext
 McMaster.Extensions.CommandLineUtils.ValidateMethodConvention.ValidateMethodConvention() -> void
 McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator
 McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.AttributeValidator(System.ComponentModel.DataAnnotations.ValidationAttribute! attribute) -> void
-McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandArgument! argument, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult!
-McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandLineApplication! command, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult!
-McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandOption! option, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult!
+McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandArgument! argument, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult?
+McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandLineApplication! command, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult?
+McMaster.Extensions.CommandLineUtils.Validation.AttributeValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandOption! option, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult?
 McMaster.Extensions.CommandLineUtils.Validation.DelegateValidator
 McMaster.Extensions.CommandLineUtils.Validation.DelegateValidator.DelegateValidator(System.Func<System.ComponentModel.DataAnnotations.ValidationContext!, System.ComponentModel.DataAnnotations.ValidationResult!>! validator) -> void
 McMaster.Extensions.CommandLineUtils.Validation.FilePathExistsAttributeBase
@@ -446,14 +446,14 @@ McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidationBuilder
 McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidationBuilder.Use(McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidator! validator) -> void
 McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidationBuilder<T>
 McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidator
-McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandArgument! argument, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult!
+McMaster.Extensions.CommandLineUtils.Validation.IArgumentValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandArgument! argument, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult?
 McMaster.Extensions.CommandLineUtils.Validation.ICommandValidator
-McMaster.Extensions.CommandLineUtils.Validation.ICommandValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandLineApplication! command, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult!
+McMaster.Extensions.CommandLineUtils.Validation.ICommandValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandLineApplication! command, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult?
 McMaster.Extensions.CommandLineUtils.Validation.IOptionValidationBuilder
 McMaster.Extensions.CommandLineUtils.Validation.IOptionValidationBuilder.Use(McMaster.Extensions.CommandLineUtils.Validation.IOptionValidator! validator) -> void
 McMaster.Extensions.CommandLineUtils.Validation.IOptionValidationBuilder<T>
 McMaster.Extensions.CommandLineUtils.Validation.IOptionValidator
-McMaster.Extensions.CommandLineUtils.Validation.IOptionValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandOption! option, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult!
+McMaster.Extensions.CommandLineUtils.Validation.IOptionValidator.GetValidationResult(McMaster.Extensions.CommandLineUtils.CommandOption! option, System.ComponentModel.DataAnnotations.ValidationContext! context) -> System.ComponentModel.DataAnnotations.ValidationResult?
 McMaster.Extensions.CommandLineUtils.Validation.IValidationBuilder
 McMaster.Extensions.CommandLineUtils.Validation.IValidationBuilder.Use(McMaster.Extensions.CommandLineUtils.Validation.IValidator! validator) -> void
 McMaster.Extensions.CommandLineUtils.Validation.IValidationBuilder<T>
diff --git a/src/CommandLineUtils/Validation/AttributeValidator.cs b/src/CommandLineUtils/Validation/AttributeValidator.cs
index 565b6ab9..5816553a 100644
--- a/src/CommandLineUtils/Validation/AttributeValidator.cs
+++ b/src/CommandLineUtils/Validation/AttributeValidator.cs
@@ -30,7 +30,7 @@ public AttributeValidator(ValidationAttribute attribute)
         /// <param name="option"></param>
         /// <param name="context"></param>
         /// <returns></returns>
-        public ValidationResult GetValidationResult(CommandOption option, ValidationContext context)
+        public ValidationResult? GetValidationResult(CommandOption option, ValidationContext context)
         {
             if (ValidationAttribute is RequiredAttribute && option.OptionType == CommandOptionType.NoValue)
             {
@@ -49,10 +49,10 @@ public ValidationResult GetValidationResult(CommandOption option, ValidationCont
         /// <param name="argument"></param>
         /// <param name="context"></param>
         /// <returns></returns>
-        public ValidationResult GetValidationResult(CommandArgument argument, ValidationContext context)
+        public ValidationResult? GetValidationResult(CommandArgument argument, ValidationContext context)
             => GetValidationResult(argument.Values, context);
 
-        private ValidationResult GetValidationResult(IReadOnlyList<string?>? values, ValidationContext context)
+        private ValidationResult? GetValidationResult(IReadOnlyList<string?>? values, ValidationContext context)
         {
             if (values == null)
             {
@@ -79,7 +79,7 @@ private ValidationResult GetValidationResult(IReadOnlyList<string?>? values, Val
         /// <summary>Checks whether the command is valid using any associated validation attributes.</summary>
         /// <param name="command">The command line application to validate</param>
         /// <param name="context">The context under which validation should be performed</param>
-        public ValidationResult GetValidationResult(CommandLineApplication command, ValidationContext context)
+        public ValidationResult? GetValidationResult(CommandLineApplication command, ValidationContext context)
         {
             var model = (command as IModelAccessor)?.GetModel();
             return model != null
diff --git a/src/CommandLineUtils/Validation/IArgumentValidator.cs b/src/CommandLineUtils/Validation/IArgumentValidator.cs
index 15f71aa1..ed50fc24 100644
--- a/src/CommandLineUtils/Validation/IArgumentValidator.cs
+++ b/src/CommandLineUtils/Validation/IArgumentValidator.cs
@@ -16,6 +16,6 @@ public interface IArgumentValidator
         /// <param name="argument">The argument.</param>
         /// <param name="context">The validation context.</param>
         /// <returns>The validation result. Returns <see cref="ValidationResult.Success"/> if the values pass validation.</returns>
-        ValidationResult GetValidationResult(CommandArgument argument, ValidationContext context);
+        ValidationResult? GetValidationResult(CommandArgument argument, ValidationContext context);
     }
 }
diff --git a/src/CommandLineUtils/Validation/ICommandValidator.cs b/src/CommandLineUtils/Validation/ICommandValidator.cs
index f14eb70c..1c9962dd 100644
--- a/src/CommandLineUtils/Validation/ICommandValidator.cs
+++ b/src/CommandLineUtils/Validation/ICommandValidator.cs
@@ -16,6 +16,6 @@ public interface ICommandValidator
         /// <param name="command">The command.</param>
         /// <param name="context">The validation context.</param>
         /// <returns>The validation result. Returns <see cref="ValidationResult.Success"/> if the values pass validation.</returns>
-        ValidationResult GetValidationResult(CommandLineApplication command, ValidationContext context);
+        ValidationResult? GetValidationResult(CommandLineApplication command, ValidationContext context);
     }
 }
diff --git a/src/CommandLineUtils/Validation/IOptionValidator.cs b/src/CommandLineUtils/Validation/IOptionValidator.cs
index e298beba..086bde12 100644
--- a/src/CommandLineUtils/Validation/IOptionValidator.cs
+++ b/src/CommandLineUtils/Validation/IOptionValidator.cs
@@ -16,6 +16,6 @@ public interface IOptionValidator
         /// <param name="option">The option.</param>
         /// <param name="context">The validation context.</param>
         /// <returns>The validation result. Returns <see cref="ValidationResult.Success"/> if the values pass validation.</returns>
-        ValidationResult GetValidationResult(CommandOption option, ValidationContext context);
+        ValidationResult? GetValidationResult(CommandOption option, ValidationContext context);
     }
 }
diff --git a/src/Hosting.CommandLine/HostBuilderExtensions.cs b/src/Hosting.CommandLine/HostBuilderExtensions.cs
index be8f4902..b27d2614 100644
--- a/src/Hosting.CommandLine/HostBuilderExtensions.cs
+++ b/src/Hosting.CommandLine/HostBuilderExtensions.cs
@@ -35,7 +35,7 @@ public static async Task<int> RunCommandLineApplicationAsync<TApp>(
             CancellationToken cancellationToken = default)
             where TApp : class
         {
-            return await RunCommandLineApplicationAsync<TApp>(hostBuilder, args, null, cancellationToken);
+            return await RunCommandLineApplicationAsync<TApp>(hostBuilder, args, _ => { }, cancellationToken);
         }
 
         /// <summary>
diff --git a/test/CommandLineUtils.Tests/CustomValidationAttributeTest.cs b/test/CommandLineUtils.Tests/CustomValidationAttributeTest.cs
index cc00e855..f6284cde 100644
--- a/test/CommandLineUtils.Tests/CustomValidationAttributeTest.cs
+++ b/test/CommandLineUtils.Tests/CustomValidationAttributeTest.cs
@@ -59,7 +59,7 @@ public RedOrBlueAttribute()
             {
             }
 
-            protected override ValidationResult IsValid(object value, ValidationContext context)
+            protected override ValidationResult? IsValid(object? value, ValidationContext context)
             {
                 if (value == null || (value is string str && str != "red" && str != "blue"))
                 {
diff --git a/test/CommandLineUtils.Tests/McMaster.Extensions.CommandLineUtils.Tests.csproj b/test/CommandLineUtils.Tests/McMaster.Extensions.CommandLineUtils.Tests.csproj
index 0a495e9f..698fd2e5 100644
--- a/test/CommandLineUtils.Tests/McMaster.Extensions.CommandLineUtils.Tests.csproj
+++ b/test/CommandLineUtils.Tests/McMaster.Extensions.CommandLineUtils.Tests.csproj
@@ -3,6 +3,8 @@
   <PropertyGroup>
     <TargetFrameworks>net5.0;netcoreapp3.1;netcoreapp2.1</TargetFrameworks>
     <TargetFrameworks Condition="'$(TestFullFramework)' != 'false'">$(TargetFrameworks);net472</TargetFrameworks>
+    <!-- Once xunit supports nullable reference types, I might reconsider -->
+    <Nullable>annotations</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/test/CommandLineUtils.Tests/ValidationTests.cs b/test/CommandLineUtils.Tests/ValidationTests.cs
index b3953cb8..83f233cb 100644
--- a/test/CommandLineUtils.Tests/ValidationTests.cs
+++ b/test/CommandLineUtils.Tests/ValidationTests.cs
@@ -149,7 +149,8 @@ public void RequiredOption_ErrorMessage(string template, string valueName)
             var app = new CommandLineApplication();
             app.Option(template, string.Empty, CommandOptionType.SingleValue).IsRequired();
             var validation = app.GetValidationResult();
-            Assert.Equal(expected, validation.ErrorMessage);
+            Assert.NotNull(validation);
+            Assert.Equal(expected, validation!.ErrorMessage);
         }
 
         [Fact]
@@ -160,7 +161,8 @@ public void RequiredArgument_ErrorMessage()
             var app = new CommandLineApplication();
             app.Argument("Arg", string.Empty).IsRequired();
             var validation = app.GetValidationResult();
-            Assert.Equal(expected, validation.ErrorMessage);
+            Assert.NotNull(validation);
+            Assert.Equal(expected, validation!.ErrorMessage);
         }
 
         [Subcommand(typeof(ValidationErrorSubcommand))]
diff --git a/test/Hosting.CommandLine.Tests/CustomValueParserTests.cs b/test/Hosting.CommandLine.Tests/CustomValueParserTests.cs
index 54a66bd1..5ef3583f 100644
--- a/test/Hosting.CommandLine.Tests/CustomValueParserTests.cs
+++ b/test/Hosting.CommandLine.Tests/CustomValueParserTests.cs
@@ -64,15 +64,17 @@ public async Task ItParsesUsingCustomParserFromAttribute()
 
         class CustomType
         {
-            public string Value { get; set; }
+            public string? Value { get; set; }
         }
 
-        class CustomValueParser : IValueParser<CustomType>
+        class CustomValueParser : IValueParser<CustomType?>
         {
             public Type TargetType => typeof(CustomType);
 
-            public CustomType Parse(string? argName, string? value, CultureInfo culture)
+            public CustomType? Parse(string? argName, string? value, CultureInfo culture)
             {
+                if (value == null)
+                    return null;
                 return JsonSerializer.Deserialize<CustomType>(value);
             }
 
@@ -86,7 +88,7 @@ public CustomType Parse(string? argName, string? value, CultureInfo culture)
         class CustomOptionTypeCommand
         {
             [Option("--custom-type", CommandOptionType.SingleValue)]
-            public CustomType Option { get; set; }
+            public CustomType? Option { get; set; }
 
             private int OnExecute()
             {
@@ -126,7 +128,7 @@ public void Apply(ConventionContext context)
         class CustomOptionTypeCommandWithAttribute
         {
             [Option("--custom-type", CommandOptionType.SingleValue)]
-            public CustomType Option { get; set; }
+            public CustomType? Option { get; set; }
 
             private int OnExecute()
             {
diff --git a/test/Hosting.CommandLine.Tests/HostBuilderExtensionsAttributeAPITests.cs b/test/Hosting.CommandLine.Tests/HostBuilderExtensionsAttributeAPITests.cs
index 9f862204..0e9581b1 100644
--- a/test/Hosting.CommandLine.Tests/HostBuilderExtensionsAttributeAPITests.cs
+++ b/test/Hosting.CommandLine.Tests/HostBuilderExtensionsAttributeAPITests.cs
@@ -69,7 +69,7 @@ public async Task TestConventionInjection()
         [Fact]
         public async Task TestConfigureCommandLineApplication()
         {
-            CommandLineApplication<Return42Command> commandLineApp = default;
+            CommandLineApplication<Return42Command>? commandLineApp = null;
             await new HostBuilder()
                 .RunCommandLineApplicationAsync<Return42Command>(
                 Array.Empty<string>(),
@@ -80,7 +80,7 @@ public async Task TestConfigureCommandLineApplication()
         [Fact]
         public async Task TestUseCommandLineApplication()
         {
-            CommandLineApplication<Return42Command> commandLineApp = default;
+            CommandLineApplication<Return42Command>? commandLineApp = null;
             var hostBuilder = new HostBuilder();
             hostBuilder.UseCommandLineApplication<Return42Command>(Array.Empty<string>(), app => commandLineApp = app);
             var host = hostBuilder.Build();
diff --git a/test/Hosting.CommandLine.Tests/HostBuilderExtensionsBuilderAPITests.cs b/test/Hosting.CommandLine.Tests/HostBuilderExtensionsBuilderAPITests.cs
index 64ae05e5..5856da8f 100644
--- a/test/Hosting.CommandLine.Tests/HostBuilderExtensionsBuilderAPITests.cs
+++ b/test/Hosting.CommandLine.Tests/HostBuilderExtensionsBuilderAPITests.cs
@@ -57,7 +57,7 @@ public async Task TestConventionInjection()
                     c.Application.UnrecognizedArgumentHandling = UnrecognizedArgumentHandling.StopParsingAndCollect)
                 .Verifiable();
             var args = new[] { "Capture", "some", "test", "arguments" };
-            string[] remainingArgs = null;
+            string[]? remainingArgs = null;
             await new HostBuilder()
                 .ConfigureServices(collection => collection
                     .AddSingleton<IConsole>(new TestConsole(_output))
@@ -96,7 +96,7 @@ public async Task ItRethrowsThrownExceptions()
         [Fact]
         public async Task TestUsingServiceProvider()
         {
-            IHostEnvironment env = null;
+            IHostEnvironment? env = null;
 
             await new HostBuilder()
                 .ConfigureServices(collection => collection.AddSingleton<IConsole>(new TestConsole(_output)))
@@ -111,8 +111,8 @@ public async Task TestUsingServiceProvider()
         [Fact]
         public async Task TestCommandLineContextFromNonDIContexts()
         {
-            CommandLineContext configureServicesContext = null;
-            CommandLineContext configureAppContext = null;
+            CommandLineContext? configureServicesContext = null;
+            CommandLineContext? configureAppContext = null;
             await new HostBuilder()
                .ConfigureServices((context, collection) =>
                {