diff --git a/src/SerializedTypes.h b/src/SerializedTypes.h
index 70e3660866..f9c68a2e15 100644
--- a/src/SerializedTypes.h
+++ b/src/SerializedTypes.h
@@ -68,7 +68,8 @@ public:
 
 	virtual void add(Serializer& s) const { return; }
 
-	virtual bool isEquivalent(const SerializedType& t) const { return true; }
+	virtual bool isEquivalent(const SerializedType& t) const
+	{ assert(getSType() == STI_NOTPRESENT); return t.getSType() == STI_NOTPRESENT; }
 
 	bool operator==(const SerializedType& t) const
 	{ return (getSType() == t.getSType()) && isEquivalent(t); }