diff --git a/src/D2L.CodeStyle.Analyzers/Helpers/ExemptSymbolsBuilder.cs b/src/D2L.CodeStyle.Analyzers/Helpers/ExemptSymbolsBuilder.cs
index d9016ddd..3b9411b9 100644
--- a/src/D2L.CodeStyle.Analyzers/Helpers/ExemptSymbolsBuilder.cs
+++ b/src/D2L.CodeStyle.Analyzers/Helpers/ExemptSymbolsBuilder.cs
@@ -11,7 +11,8 @@ internal sealed class ExemptSymbolsBuilder {
 	private readonly AnalyzerOptions m_analyzerOptions;
 	private readonly CancellationToken m_cancellationToken;
 
-	private readonly ImmutableHashSet<ISymbol>.Builder m_exemptions = ImmutableHashSet.CreateBuilder<ISymbol>( SymbolEqualityComparer.Default );
+	private readonly HashSet<ISymbol> m_exemptions = new( SymbolEqualityComparer.Default );
+	private bool m_built = false;
 
 	[System.Diagnostics.CodeAnalysis.SuppressMessage(
 		"MicrosoftCodeAnalysisPerformance",
@@ -26,7 +27,12 @@ CompilationStartAnalysisContext context
 		m_cancellationToken = context.CancellationToken;
 	}
 
-	public ImmutableHashSet<ISymbol> Build() => m_exemptions.ToImmutable();
+	public HashSet<ISymbol> Build() {
+		ThrowIfBuilt();
+
+		m_built = true;
+		return m_exemptions;
+	}
 
 	/// <summary>
 	/// Loads exemptions from AdditionalFiles matching "<paramref name="fileNameBase"/>.txt" and "<paramref name="fileNameBase"/>.*.txt".
@@ -35,6 +41,8 @@ CompilationStartAnalysisContext context
 	public ExemptSymbolsBuilder AddFromAdditionalFiles(
 		string fileNameBase
 	) {
+		ThrowIfBuilt();
+
 		fileNameBase += ".";
 
 		foreach( AdditionalText file in m_analyzerOptions.AdditionalFiles ) {
@@ -73,6 +81,8 @@ string fileNameBase
 	}
 
 	public ExemptSymbolsBuilder AddFromDocumentationCommentId( string id ) {
+		ThrowIfBuilt();
+
 		ImmutableArray<ISymbol> symbols = DocumentationCommentId.GetSymbolsForDeclarationId( id, m_compilation );
 		foreach( var symbol in symbols ) {
 			m_exemptions.Add( symbol );
@@ -81,4 +91,10 @@ public ExemptSymbolsBuilder AddFromDocumentationCommentId( string id ) {
 		return this;
 	}
 
+	private void ThrowIfBuilt() {
+		if( m_built ) {
+			throw new InvalidOperationException( "Builder instance has already been used" );
+		}
+	}
+
 }
diff --git a/src/D2L.CodeStyle.Analyzers/Language/RequireNamedArgumentsAnalyzer.cs b/src/D2L.CodeStyle.Analyzers/Language/RequireNamedArgumentsAnalyzer.cs
index 74c758e4..4d3cf214 100644
--- a/src/D2L.CodeStyle.Analyzers/Language/RequireNamedArgumentsAnalyzer.cs
+++ b/src/D2L.CodeStyle.Analyzers/Language/RequireNamedArgumentsAnalyzer.cs
@@ -53,7 +53,7 @@ private static void OnCompilationStart( CompilationStartAnalysisContext context
 				return;
 			}
 
-			ImmutableHashSet<ISymbol> exemptions = new ExemptSymbolsBuilder( context )
+			HashSet<ISymbol> exemptions = new ExemptSymbolsBuilder( context )
 				.AddFromAdditionalFiles( "D2L.CodeStyle.RequireNamedArguments.Exemptions" )
 
 				// HashCode.Combine takes a series of args named value1..valueN which are not useful to name
@@ -86,7 +86,7 @@ private static void AnalyzeInvocation(
 			OperationAnalysisContext ctx,
 			INamedTypeSymbol requireNamedArgumentsSymbol,
 			INamedTypeSymbol? lambdaExpresssionSymbol,
-			ImmutableHashSet<ISymbol> exemptions
+			HashSet<ISymbol> exemptions
 		) {
 			(IMethodSymbol targetMethod, ImmutableArray<IArgumentOperation> args) = ctx.Operation switch {
 				IInvocationOperation op => (op.TargetMethod, op.Arguments),