diff --git a/Directory.Build.props b/Directory.Build.props
index 86609829..c1307957 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -4,5 +4,6 @@
 		<LangVersion>10.0</LangVersion>
 		<ImplicitUsings>true</ImplicitUsings>
 		<InvariantGlobalization>true</InvariantGlobalization>
+		<Nullable>enable</Nullable>
 	</PropertyGroup>
 </Project>
diff --git a/src/D2L.CodeStyle.Analyzers/D2L.CodeStyle.Analyzers.csproj b/src/D2L.CodeStyle.Analyzers/D2L.CodeStyle.Analyzers.csproj
index 1d0b724d..4cd61a0b 100644
--- a/src/D2L.CodeStyle.Analyzers/D2L.CodeStyle.Analyzers.csproj
+++ b/src/D2L.CodeStyle.Analyzers/D2L.CodeStyle.Analyzers.csproj
@@ -17,7 +17,6 @@
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <IncludeBuildOutput>False</IncludeBuildOutput>
     <IncludeSymbols>True</IncludeSymbols>
-    <Nullable>enable</Nullable>
     <WarningsAsErrors>CA2016,Nullable</WarningsAsErrors>
     <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
     <EnforceExtendedAnalyzerRules>true</EnforceExtendedAnalyzerRules>
diff --git a/src/D2L.CodeStyle.TestAnalyzers/Common/NotNullWhenAttribute.cs b/src/D2L.CodeStyle.TestAnalyzers/Common/NotNullWhenAttribute.cs
new file mode 100644
index 00000000..461512e8
--- /dev/null
+++ b/src/D2L.CodeStyle.TestAnalyzers/Common/NotNullWhenAttribute.cs
@@ -0,0 +1,14 @@
+#if !NETSTANDARD2_1
+
+namespace System.Diagnostics.CodeAnalysis;
+
+[AttributeUsage( AttributeTargets.Parameter )]
+internal sealed class NotNullWhenAttribute : Attribute {
+	public bool ReturnValue { get; }
+
+	public NotNullWhenAttribute( bool returnValue ) {
+		ReturnValue = returnValue;
+	}
+}
+
+#endif
diff --git a/src/D2L.CodeStyle.TestAnalyzers/Extensions/Microsoft.CodeAnalysis.cs b/src/D2L.CodeStyle.TestAnalyzers/Extensions/Microsoft.CodeAnalysis.cs
index d4c6c59e..059370c2 100644
--- a/src/D2L.CodeStyle.TestAnalyzers/Extensions/Microsoft.CodeAnalysis.cs
+++ b/src/D2L.CodeStyle.TestAnalyzers/Extensions/Microsoft.CodeAnalysis.cs
@@ -9,7 +9,7 @@ public static class RoslynExtensions {
 
 		// Copied from the non-test assembly because we do not reference it.
 
-		public static bool IsNullOrErrorType( this ITypeSymbol symbol ) {
+		public static bool IsNullOrErrorType( this ITypeSymbol? symbol ) {
 
 			if( symbol == null ) {
 				return true;
@@ -26,6 +26,19 @@ public static bool IsNullOrErrorType( this ITypeSymbol symbol ) {
 			return false;
 		}
 
+		public static bool IsErrorType( this ITypeSymbol symbol ) {
+
+			if( symbol.Kind == SymbolKind.ErrorType ) {
+				return true;
+			}
+
+			if( symbol.TypeKind == TypeKind.Error ) {
+				return true;
+			}
+
+			return false;
+		}
+
 		public static bool IsNullOrErrorType( this ISymbol symbol ) {
 
 			if( symbol == null ) {
diff --git a/src/D2L.CodeStyle.TestAnalyzers/NUnit/CategoryAnalyzer.cs b/src/D2L.CodeStyle.TestAnalyzers/NUnit/CategoryAnalyzer.cs
index faf8fab8..b5c5582c 100644
--- a/src/D2L.CodeStyle.TestAnalyzers/NUnit/CategoryAnalyzer.cs
+++ b/src/D2L.CodeStyle.TestAnalyzers/NUnit/CategoryAnalyzer.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using D2L.CodeStyle.TestAnalyzers.Common;
@@ -42,7 +43,7 @@ public override void Initialize( AnalysisContext context ) {
 		}
 
 		private static void OnCompilationStart( CompilationStartAnalysisContext context ) {
-			if( !TryLoadNUnitTypes( context.Compilation, out NUnitTypes types ) ) {
+			if( !TryLoadNUnitTypes( context.Compilation, out NUnitTypes? types ) ) {
 				return;
 			}
 
@@ -53,7 +54,7 @@ private static void OnCompilationStart( CompilationStartAnalysisContext context
 					context: ctx,
 					bannedCategories: bannedCategories,
 					types: types,
-					syntax: ctx.Node as MethodDeclarationSyntax
+					syntax: (ctx.Node as MethodDeclarationSyntax)!
 				),
 				SyntaxKind.MethodDeclaration
 			);
@@ -69,9 +70,16 @@ private static void OnCompilationEnd( CompilationAnalysisContext context, NUnitT
 					return;
 				}
 
+				var syntaxReference = attribute.ApplicationSyntaxReference;
+
+				if( syntaxReference == null ) {
+					// ??
+					return;
+				}
+
 				context.ReportDiagnostic( Diagnostic.Create(
 					Diagnostics.NUnitCategory,
-					attribute.ApplicationSyntaxReference.GetSyntax( context.CancellationToken ).GetLocation(),
+					syntaxReference.GetSyntax( context.CancellationToken ).GetLocation(),
 					$"Assemblies cannot be categorized as any of [{string.Join( ", ", ProhibitedAssemblyCategories )}], but saw '{category}'."
 				) );
 			} );
@@ -85,7 +93,7 @@ MethodDeclarationSyntax syntax
 		) {
 			SemanticModel model = context.SemanticModel;
 
-			IMethodSymbol method = model.GetDeclaredSymbol( syntax, context.CancellationToken );
+			IMethodSymbol? method = model.GetDeclaredSymbol( syntax, context.CancellationToken );
 			if( method == null ) {
 				return;
 			}
@@ -117,8 +125,8 @@ private static bool IsTestMethod(
 			IMethodSymbol method
 		) {
 			foreach( AttributeData attribute in method.GetAttributes() ) {
-				INamedTypeSymbol attributeType = attribute.AttributeClass;
-				if( types.TestAttributes.Contains( attributeType ) ) {
+				INamedTypeSymbol? attributeType = attribute.AttributeClass;
+				if( attributeType != null && types.TestAttributes.Contains( attributeType ) ) {
 					return true;
 				}
 			}
@@ -159,7 +167,7 @@ private static void VisitCategories(
 			Action<string, AttributeData> visitor
 		) {
 			foreach( AttributeData attribute in symbol.GetAttributes() ) {
-				INamedTypeSymbol attributeType = attribute.AttributeClass;
+				INamedTypeSymbol? attributeType = attribute.AttributeClass;
 				if( types.CategoryAttribute.Equals( attributeType, SymbolEqualityComparer.Default ) ) {
 					VisitCategoryAttribute( attribute, visitor );
 					continue;
@@ -189,11 +197,16 @@ Action<string, AttributeData> visitor
 
 			TypedConstant arg = args[0];
 
-			if( arg.Type.SpecialType != SpecialType.System_String ) {
+			if( arg.Type?.SpecialType != SpecialType.System_String ) {
+				return;
+			}
+
+			string? category = arg.Value as string;
+
+			if( category == null ) {
 				return;
 			}
 
-			string category = arg.Value as string;
 			visitor( category, attribute );
 		}
 
@@ -208,11 +221,16 @@ Action<string, AttributeData> visitor
 
 				TypedConstant arg = namedArg.Value;
 
-				if( arg.Type.SpecialType != SpecialType.System_String ) {
+				if( arg.Type?.SpecialType != SpecialType.System_String ) {
+					continue;
+				}
+
+				string? categoryCsv = arg.Value as string;
+
+				if( categoryCsv == null ) {
 					continue;
 				}
 
-				string categoryCsv = arg.Value as string;
 				foreach( string category in categoryCsv.Split( ',' ) ) {
 					visitor( category.Trim(), attribute );
 				}
@@ -221,24 +239,29 @@ Action<string, AttributeData> visitor
 
 		private static bool TryLoadNUnitTypes(
 			Compilation compilation,
-			out NUnitTypes types
+			[NotNullWhen( true )]
+			out NUnitTypes? types
 		) {
-			INamedTypeSymbol categoryAttribute = compilation.GetTypeByMetadataName( "NUnit.Framework.CategoryAttribute" );
+			INamedTypeSymbol? categoryAttribute = compilation.GetTypeByMetadataName( "NUnit.Framework.CategoryAttribute" );
 			if( categoryAttribute == null || categoryAttribute.TypeKind == TypeKind.Error ) {
 				types = null;
 				return false;
 			}
 
-			ImmutableHashSet<INamedTypeSymbol> testAttributes = ImmutableHashSet
-				.Create<INamedTypeSymbol>(
-					SymbolEqualityComparer.Default,
-					compilation.GetTypeByMetadataName( "NUnit.Framework.TestAttribute" ),
-					compilation.GetTypeByMetadataName( "NUnit.Framework.TestCaseAttribute" ),
-					compilation.GetTypeByMetadataName( "NUnit.Framework.TestCaseSourceAttribute" ),
-					compilation.GetTypeByMetadataName( "NUnit.Framework.TheoryAttribute" )
-				);
+			ImmutableHashSet<INamedTypeSymbol> testAttributes = new[] {
+				compilation.GetTypeByMetadataName( "NUnit.Framework.TestAttribute" ),
+				compilation.GetTypeByMetadataName( "NUnit.Framework.TestCaseAttribute" ),
+				compilation.GetTypeByMetadataName( "NUnit.Framework.TestCaseSourceAttribute" ),
+				compilation.GetTypeByMetadataName( "NUnit.Framework.TheoryAttribute" )
+			}.Where( x => x != null )!
+			.ToImmutableHashSet<INamedTypeSymbol>( SymbolEqualityComparer.Default );
 
-			INamedTypeSymbol testFixtureAttribute = compilation.GetTypeByMetadataName( "NUnit.Framework.TestFixtureAttribute" );
+			INamedTypeSymbol? testFixtureAttribute = compilation.GetTypeByMetadataName( "NUnit.Framework.TestFixtureAttribute" );
+
+			if( testFixtureAttribute == null ) {
+				types = null;
+				return false;
+			}
 
 			types = new NUnitTypes( categoryAttribute, testAttributes, testFixtureAttribute );
 			return true;
@@ -268,7 +291,7 @@ AnalyzerOptions options
 				StringComparer.Ordinal
 			);
 
-			AdditionalText bannedListFile = options.AdditionalFiles.FirstOrDefault(
+			AdditionalText? bannedListFile = options.AdditionalFiles.FirstOrDefault(
 				file => Path.GetFileName( file.Path ) == "BannedTestCategoriesList.txt"
 			);
 
@@ -276,7 +299,11 @@ AnalyzerOptions options
 				return bannedList.ToImmutableHashSet();
 			}
 
-			SourceText allowedListText = bannedListFile.GetText();
+			SourceText? allowedListText = bannedListFile.GetText();
+
+			if( allowedListText == null ) {
+				throw new Exception( "Couldn't read config" );
+			}
 
 			foreach( TextLine line in allowedListText.Lines ) {
 				bannedList.Add( line.ToString().Trim() );
diff --git a/src/D2L.CodeStyle.TestAnalyzers/NUnit/ConfigTestSetupStringsAnalyzer.cs b/src/D2L.CodeStyle.TestAnalyzers/NUnit/ConfigTestSetupStringsAnalyzer.cs
index 44382406..6828a5c6 100644
--- a/src/D2L.CodeStyle.TestAnalyzers/NUnit/ConfigTestSetupStringsAnalyzer.cs
+++ b/src/D2L.CodeStyle.TestAnalyzers/NUnit/ConfigTestSetupStringsAnalyzer.cs
@@ -27,10 +27,10 @@ public override void Initialize( AnalysisContext context ) {
 
 		private static void Register( CompilationStartAnalysisContext context ) {
 
-			INamedTypeSymbol attributeType =
+			INamedTypeSymbol? attributeType =
 				context.Compilation.GetTypeByMetadataName( AttributeTypeName );
 
-			if( attributeType.IsNullOrErrorType() ) {
+			if( attributeType == null || attributeType.IsErrorType() ) {
 				return;
 			}
 
@@ -73,12 +73,12 @@ INamedTypeSymbol attributeType
 
 			foreach( AttributeSyntax attribute in attributes ) {
 
-				ISymbol symbol = context
+				ISymbol? symbol = context
 					.SemanticModel
 					.GetSymbolInfo( attribute, context.CancellationToken )
 					.Symbol;
 
-				if( symbol.IsNullOrErrorType() ) {
+				if( symbol == null || symbol.Kind == SymbolKind.ErrorType ) {
 					continue;
 				}
 
@@ -88,9 +88,9 @@ INamedTypeSymbol attributeType
 				}
 
 				SeparatedSyntaxList<AttributeArgumentSyntax> arguments =
-					attribute.ArgumentList.Arguments;
+					attribute.ArgumentList?.Arguments ?? default;
 
-				if( arguments.Count != 1 ) {
+				if( arguments == default || arguments.Count != 1 ) {
 					continue;
 				}
 
diff --git a/src/D2L.CodeStyle.TestAnalyzers/ServiceLocator/CustomTestServiceLocatorAnalyzer.cs b/src/D2L.CodeStyle.TestAnalyzers/ServiceLocator/CustomTestServiceLocatorAnalyzer.cs
index 421aef3e..c07cc0a5 100644
--- a/src/D2L.CodeStyle.TestAnalyzers/ServiceLocator/CustomTestServiceLocatorAnalyzer.cs
+++ b/src/D2L.CodeStyle.TestAnalyzers/ServiceLocator/CustomTestServiceLocatorAnalyzer.cs
@@ -28,10 +28,10 @@ public override void Initialize( AnalysisContext context ) {
 		public static void RegisterServiceLocatorAnalyzer(
 			CompilationStartAnalysisContext context
 		) {
-			INamedTypeSymbol factoryType = context.Compilation
+			INamedTypeSymbol? factoryType = context.Compilation
 				.GetTypeByMetadataName( TestServiceLocatorFactoryType );
 
-			if( factoryType.IsNullOrErrorType() ) {
+			if( factoryType == null || factoryType.IsErrorType() ) {
 				return;
 			}