diff --git a/src/libraries/Directory.Build.targets b/src/libraries/Directory.Build.targets
index cad6c2ed4803af..6ac85438567d03 100644
--- a/src/libraries/Directory.Build.targets
+++ b/src/libraries/Directory.Build.targets
@@ -6,7 +6,7 @@
   <Import Project="..\..\Directory.Build.targets" />
 
   <PropertyGroup>
-    <NoWarn Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework' or '$(TargetFrameworkIdentifier)' == '.NETStandard'">$(NoWarn);nullable</NoWarn>
+    <NoWarn Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework' or '$(TargetFrameworkIdentifier)' == '.NETStandard' or '$(TargetFramework)' == 'netcoreapp2.1'">$(NoWarn);nullable</NoWarn>
     <!-- Reset these properties back to blank, since they are defaulted by Microsoft.NET.Sdk -->
     <FileAlignment Condition="'$(FileAlignment)' == '512'" />
     <ErrorReport Condition="'$(ErrorReport)' == 'prompt'" />
diff --git a/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.cs b/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.cs
index ca291e4fc7a092..892c8b85560f60 100644
--- a/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.cs
+++ b/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.cs
@@ -9,6 +9,6 @@ namespace System.Net.WebSockets
 {
     public static partial class WebSocketProtocol
     {
-        public static System.Net.WebSockets.WebSocket CreateFromStream(System.IO.Stream stream, bool isServer, string subProtocol, System.TimeSpan keepAliveInterval) { throw null; }
+        public static System.Net.WebSockets.WebSocket CreateFromStream(System.IO.Stream stream, bool isServer, string? subProtocol, System.TimeSpan keepAliveInterval) { throw null; }
     }
 }
diff --git a/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.csproj b/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.csproj
index 858e8bf204b16d..4c1d519edd7c3c 100644
--- a/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.csproj
+++ b/src/libraries/System.Net.WebSockets.WebSocketProtocol/ref/System.Net.WebSockets.WebSocketProtocol.csproj
@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Net.WebSockets.WebSocketProtocol.cs" />
diff --git a/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System.Net.WebSockets.WebSocketProtocol.csproj b/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System.Net.WebSockets.WebSocketProtocol.csproj
index a85b95b6c768d6..21bf787ade93cf 100644
--- a/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System.Net.WebSockets.WebSocketProtocol.csproj
+++ b/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System.Net.WebSockets.WebSocketProtocol.csproj
@@ -5,7 +5,7 @@
     <NoWarn>$(NoWarn);CS1573</NoWarn>
     <TargetFrameworks>$(NetCoreAppCurrent);netstandard2.0;netcoreapp2.1</TargetFrameworks>
     <ExcludeCurrentNetCoreAppFromPackage>true</ExcludeCurrentNetCoreAppFromPackage>
-    <Nullable>annotations</Nullable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonPath)System\Net\WebSockets\ManagedWebSocket.cs"
diff --git a/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/ManagedWebSocketExtensions.cs b/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/ManagedWebSocketExtensions.cs
index 7eb3997a65b264..e5bb310ce59d2f 100644
--- a/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/ManagedWebSocketExtensions.cs
+++ b/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/ManagedWebSocketExtensions.cs
@@ -8,7 +8,7 @@ namespace System.Net.WebSockets
 {
     internal static partial class ManagedWebSocketExtensions
     {
-        internal static CancellationTokenRegistration UnsafeRegister(this CancellationToken cancellationToken, Action<object> callback, object state) =>
+        internal static CancellationTokenRegistration UnsafeRegister(this CancellationToken cancellationToken, Action<object?> callback, object? state) =>
             cancellationToken.Register(callback, state);
     }
 }
diff --git a/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/WebSocketProtocol.cs b/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/WebSocketProtocol.cs
index 732cbb5e913076..12e0f3656f1764 100644
--- a/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/WebSocketProtocol.cs
+++ b/src/libraries/System.Net.WebSockets.WebSocketProtocol/src/System/Net/WebSockets/WebSocketProtocol.cs
@@ -12,7 +12,7 @@ public static class WebSocketProtocol
         public static WebSocket CreateFromStream(
             Stream stream,
             bool isServer,
-            string subProtocol,
+            string? subProtocol,
             TimeSpan keepAliveInterval)
         {
             if (stream == null)
diff --git a/src/libraries/System.Net.WebSockets/ref/System.Net.WebSockets.cs b/src/libraries/System.Net.WebSockets/ref/System.Net.WebSockets.cs
index 02392bfd22d260..f9acf9d8b17861 100644
--- a/src/libraries/System.Net.WebSockets/ref/System.Net.WebSockets.cs
+++ b/src/libraries/System.Net.WebSockets/ref/System.Net.WebSockets.cs
@@ -115,7 +115,7 @@ public enum WebSocketMessageType
     public partial class WebSocketReceiveResult
     {
         public WebSocketReceiveResult(int count, System.Net.WebSockets.WebSocketMessageType messageType, bool endOfMessage) { }
-        public WebSocketReceiveResult(int count, System.Net.WebSockets.WebSocketMessageType messageType, bool endOfMessage, System.Net.WebSockets.WebSocketCloseStatus? closeStatus, string closeStatusDescription) { }
+        public WebSocketReceiveResult(int count, System.Net.WebSockets.WebSocketMessageType messageType, bool endOfMessage, System.Net.WebSockets.WebSocketCloseStatus? closeStatus, string? closeStatusDescription) { }
         public System.Net.WebSockets.WebSocketCloseStatus? CloseStatus { get { throw null; } }
         public string? CloseStatusDescription { get { throw null; } }
         public int Count { get { throw null; } }