diff --git a/eng/targets/Imports.targets b/eng/targets/Imports.targets
index ab481cf93378f..f3dd6a232b286 100644
--- a/eng/targets/Imports.targets
+++ b/eng/targets/Imports.targets
@@ -106,6 +106,9 @@
       Updating a reference to this package to a newer version of the package may require changes in the referencing project.
       No compatibility guarantees are provided.
     </PackageDescription>
+    <!-- Disable project-level nullable to catch packaged files without an explicit directive;
+      consumers may compile them in projects where nullable is disabled.  -->
+    <Nullable>disable</Nullable>
   </PropertyGroup>
 
   <!-- Include SourcePackage.editorconfig in all source packages. -->
diff --git a/src/Compilers/Shared/IBuildEnvironment.cs b/src/Compilers/Shared/IBuildEnvironment.cs
index 0a55b1982caba..92459123fcffd 100644
--- a/src/Compilers/Shared/IBuildEnvironment.cs
+++ b/src/Compilers/Shared/IBuildEnvironment.cs
@@ -3,6 +3,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.IO;
diff --git a/src/Compilers/Shared/StandardBuildEnvironment.cs b/src/Compilers/Shared/StandardBuildEnvironment.cs
index 8125928323580..8a91e9754d4f2 100644
--- a/src/Compilers/Shared/StandardBuildEnvironment.cs
+++ b/src/Compilers/Shared/StandardBuildEnvironment.cs
@@ -3,6 +3,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.IO;
diff --git a/src/NuGet/Directory.Build.props b/src/NuGet/Directory.Build.props
index 13fab30f50f0f..7382bb4463c83 100644
--- a/src/NuGet/Directory.Build.props
+++ b/src/NuGet/Directory.Build.props
@@ -1,5 +1,5 @@
 <!-- Licensed to the .NET Foundation under one or more agreements. The .NET Foundation licenses this file to you under the MIT license. See the LICENSE file in the project root for more information. -->
 <Project>
   <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))" />
-  <Import Project="$(RepositoryEngineeringDir)targets\PackageProject.props"/>
+  <Import Project="$(RepositoryEngineeringDir)targets\PackageProject.props" Condition="'$(IsSourcePackage)' != 'true'"/>
 </Project>
diff --git a/src/NuGet/Directory.Build.targets b/src/NuGet/Directory.Build.targets
index e1dbd218d1d8b..963460dafc054 100644
--- a/src/NuGet/Directory.Build.targets
+++ b/src/NuGet/Directory.Build.targets
@@ -1,5 +1,5 @@
 <!-- Licensed to the .NET Foundation under one or more agreements. The .NET Foundation licenses this file to you under the MIT license. See the LICENSE file in the project root for more information. -->
 <Project>
   <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.targets', '$(MSBuildThisFileDirectory)../'))" />
-  <Import Project="$(RepositoryEngineeringDir)targets\PackageProject.targets" Condition="'$(TargetFramework)' != ''"/>
+  <Import Project="$(RepositoryEngineeringDir)targets\PackageProject.targets" Condition="'$(IsSourcePackage)' != 'true' and '$(TargetFramework)' != ''"/>
 </Project>
\ No newline at end of file