diff --git a/src/Compilers/Core/Portable/NativePdbWriter/PdbWriter.cs b/src/Compilers/Core/Portable/NativePdbWriter/PdbWriter.cs
index 612ee3d05abde09f3e641cbe0d64c6ddd2a832c0..bd55c4a1de9be5da28a1bea4ec1b8461ec5a9262 100644
--- a/src/Compilers/Core/Portable/NativePdbWriter/PdbWriter.cs
+++ b/src/Compilers/Core/Portable/NativePdbWriter/PdbWriter.cs
@@ -624,6 +624,9 @@ private string SerializeVisualBasicImportTypeReference(ITypeReference typeRefere
 
         private string GetAssemblyReferenceAlias(IAssemblyReference assembly, HashSet<string> declaredExternAliases)
         {
+            // no extern alias defined in scope at all -> error in compiler
+            Debug.Assert(declaredExternAliases != null);
+
             var allAliases = _metadataWriter.Context.Module.GetAssemblyReferenceAliases(_metadataWriter.Context);
             foreach (AssemblyReferenceAlias alias in allAliases)
             {