diff --git a/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs b/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs
index 8e7922292..878edf8fe 100644
--- a/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs
+++ b/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs
@@ -17,6 +17,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		static readonly Decompiler.TypeSystem.GenericContext genericContext = default;
 
 		readonly HashSet<string> namespaces;
+		readonly HashSet<IType> visitedTypes = new HashSet<IType>();
 
 		public RequiredNamespaceCollector(HashSet<string> namespaces)
 		{
@@ -141,13 +142,15 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		void CollectNamespacesForTypeReference(IType type)
 		{
+			if (!visitedTypes.Add(type))
+				return;
 			switch (type) {
 				case ParameterizedType parameterizedType:
 					namespaces.Add(parameterizedType.Namespace);
 					CollectNamespacesForTypeReference(parameterizedType.GenericType);
 					foreach (var arg in parameterizedType.TypeArguments)
 						CollectNamespacesForTypeReference(arg);
-					break;
+					return; // no need to collect base types again
 				case TypeWithElementType typeWithElementType:
 					CollectNamespacesForTypeReference(typeWithElementType.ElementType);
 					break;