diff --git a/src/Core/References/NugetPackages.cs b/src/Core/References/NugetPackages.cs
index 3a26cc2be1..94b1550505 100644
--- a/src/Core/References/NugetPackages.cs
+++ b/src/Core/References/NugetPackages.cs
@@ -208,8 +208,11 @@ public async Task Add(PackageIdentity pkgId, Action<string>? statusCallback = nu
 
                 await DownloadPackages(sourceCacheContext, packages, statusCallback);
 
-                this.Items = Items.Union(new PackageIdentity[] { pkgId }).ToArray();
-                this.Assemblies = Assemblies.Union(packages.Reverse().SelectMany(GetAssemblies)).ToArray();
+                lock (this)
+                {
+                    this.Items = Items.Union(new PackageIdentity[] { pkgId }).ToArray();
+                    this.Assemblies = Assemblies.Union(packages.Reverse().SelectMany(GetAssemblies)).ToArray();
+                }
             }
         }
 
diff --git a/src/Core/References/References.cs b/src/Core/References/References.cs
index 8b1590ebb7..bd430fba83 100644
--- a/src/Core/References/References.cs
+++ b/src/Core/References/References.cs
@@ -183,8 +183,11 @@ IOptions<Settings> options
         /// </summary>
         public void AddAssemblies(params AssemblyInfo[] assemblies)
         {
-            Assemblies = Assemblies.Union(assemblies).ToImmutableArray();
-            Reset();
+            lock (this)
+            {
+                Assemblies = Assemblies.Union(assemblies).ToImmutableArray();
+                Reset();
+            }
         }
 
         /// <summary>