diff --git a/src/Hydrogen/Collections/StreamMapped/StreamMappedFactory.cs b/src/Hydrogen/Collections/StreamMapped/StreamMappedFactory.cs
index a75f1701..716ee0af 100644
--- a/src/Hydrogen/Collections/StreamMapped/StreamMappedFactory.cs
+++ b/src/Hydrogen/Collections/StreamMapped/StreamMappedFactory.cs
@@ -159,7 +159,7 @@ public static IStreamMappedDictionary<TKey, TValue> CreateDictionary<TKey, TValu
 		valueComparer ??= EqualityComparer<TValue>.Default;
 
 		// ensure it can serialize null's
-		valueSerializer = valueSerializer.AsReferenceSerializer();
+		Guard.Ensure(valueSerializer.SupportsNull, $"Value serializer {keySerializer} does not support null values");
 
 		var useCLK = false;
 		switch (implementation) {