diff --git a/csharp/extractor/Semmle.Extraction.Tests/Layout.cs b/csharp/extractor/Semmle.Extraction.Tests/Layout.cs
index 49da0fd45c53..c66b804f3a71 100644
--- a/csharp/extractor/Semmle.Extraction.Tests/Layout.cs
+++ b/csharp/extractor/Semmle.Extraction.Tests/Layout.cs
@@ -15,6 +15,8 @@ public void TestDefaultLayout()
             var layout = new Semmle.Extraction.Layout(null, null, null);
             var project = layout.LookupProjectOrNull("foo.cs");
 
+            Assert.NotNull(project);
+
             // All files are mapped when there's no layout file.
             Assert.True(layout.FileInLayout("foo.cs"));
 
@@ -28,7 +30,7 @@ public void TestDefaultLayout()
                 Assert.NotEqual(Directory.GetCurrentDirectory(), tmpDir);
                 return;
             }
-            var f1 = project.GetTrapPath(Logger, "foo.cs", TrapWriter.CompressionMode.Gzip);
+            var f1 = project!.GetTrapPath(Logger, "foo.cs", TrapWriter.CompressionMode.Gzip);
             var g1 = TrapWriter.NestPaths(Logger, tmpDir, "foo.cs.trap.gz", TrapWriter.InnerPathComputation.ABSOLUTE);
             Assert.Equal(f1, g1);
 
@@ -72,7 +74,9 @@ public void TestLayoutFile()
 
             // Test the trap file
             var project = layout.LookupProjectOrNull("bar.cs");
-            var trapwriterFilename = project.GetTrapPath(Logger, "bar.cs", TrapWriter.CompressionMode.Gzip);
+            Assert.NotNull(project);
+
+            var trapwriterFilename = project!.GetTrapPath(Logger, "bar.cs", TrapWriter.CompressionMode.Gzip);
             Assert.Equal(TrapWriter.NestPaths(Logger, Path.GetFullPath("snapshot\\trap"), "bar.cs.trap.gz", TrapWriter.InnerPathComputation.ABSOLUTE),
                 trapwriterFilename);
 
@@ -90,7 +94,9 @@ public void TestTrapOverridesLayout()
             // When you specify both a trap file and a layout, use the trap file.
             var layout = new Semmle.Extraction.Layout(Path.GetFullPath("snapshot\\trap"), null, "something.txt");
             Assert.True(layout.FileInLayout("bar.cs"));
-            var f1 = layout.LookupProjectOrNull("foo.cs").GetTrapPath(Logger, "foo.cs", TrapWriter.CompressionMode.Gzip);
+            var subProject = layout.LookupProjectOrNull("foo.cs");
+            Assert.NotNull(subProject);
+            var f1 = subProject!.GetTrapPath(Logger, "foo.cs", TrapWriter.CompressionMode.Gzip);
             var g1 = TrapWriter.NestPaths(Logger, Path.GetFullPath("snapshot\\trap"), "foo.cs.trap.gz", TrapWriter.InnerPathComputation.ABSOLUTE);
             Assert.Equal(f1, g1);
         }
@@ -118,13 +124,17 @@ public void TestMultipleSections()
 
             // Use Section 2
             Assert.True(layout.FileInLayout("bar.cs"));
-            var f1 = layout.LookupProjectOrNull("bar.cs").GetTrapPath(Logger, "bar.cs", TrapWriter.CompressionMode.Gzip);
+            var subProject = layout.LookupProjectOrNull("bar.cs");
+            Assert.NotNull(subProject);
+            var f1 = subProject!.GetTrapPath(Logger, "bar.cs", TrapWriter.CompressionMode.Gzip);
             var g1 = TrapWriter.NestPaths(Logger, Path.GetFullPath("snapshot\\trap2"), "bar.cs.trap.gz", TrapWriter.InnerPathComputation.ABSOLUTE);
             Assert.Equal(f1, g1);
 
             // Use Section 1
             Assert.True(layout.FileInLayout("foo.cs"));
-            var f2 = layout.LookupProjectOrNull("foo.cs").GetTrapPath(Logger, "foo.cs", TrapWriter.CompressionMode.Gzip);
+            subProject = layout.LookupProjectOrNull("foo.cs");
+            Assert.NotNull(subProject);
+            var f2 = subProject!.GetTrapPath(Logger, "foo.cs", TrapWriter.CompressionMode.Gzip);
             var g2 = TrapWriter.NestPaths(Logger, Path.GetFullPath("snapshot\\trap1"), "foo.cs.trap.gz", TrapWriter.InnerPathComputation.ABSOLUTE);
             Assert.Equal(f2, g2);
 
diff --git a/csharp/extractor/Semmle.Extraction.Tests/Options.cs b/csharp/extractor/Semmle.Extraction.Tests/Options.cs
index afb6df2e9ba5..ab0f099fa4c5 100644
--- a/csharp/extractor/Semmle.Extraction.Tests/Options.cs
+++ b/csharp/extractor/Semmle.Extraction.Tests/Options.cs
@@ -9,8 +9,8 @@ namespace Semmle.Extraction.Tests
 {
     public class OptionsTests
     {
-        CSharp.Options options;
-        CSharp.Standalone.Options standaloneOptions;
+        CSharp.Options? options;
+        CSharp.Standalone.Options? standaloneOptions;
 
         public OptionsTests()
         {
diff --git a/csharp/extractor/Semmle.Extraction.Tests/Semmle.Extraction.Tests.csproj b/csharp/extractor/Semmle.Extraction.Tests/Semmle.Extraction.Tests.csproj
index c101a5fba830..64980e797cd8 100644
--- a/csharp/extractor/Semmle.Extraction.Tests/Semmle.Extraction.Tests.csproj
+++ b/csharp/extractor/Semmle.Extraction.Tests/Semmle.Extraction.Tests.csproj
@@ -5,6 +5,7 @@
     <TargetFramework>netcoreapp3.0</TargetFramework>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
     <RuntimeIdentifiers>win-x64;linux-x64;osx-x64</RuntimeIdentifiers>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>