diff --git a/src/scripting/decorate/thingdef_parse.cpp b/src/scripting/decorate/thingdef_parse.cpp
index bad24c872..43330d58f 100644
--- a/src/scripting/decorate/thingdef_parse.cpp
+++ b/src/scripting/decorate/thingdef_parse.cpp
@@ -62,7 +62,14 @@ EXTERN_CVAR(Bool, strictdecorate);
 
 PClassActor *DecoDerivedClass(const FScriptPosition &sc, PClassActor *parent, FName typeName)
 {
-	if (parent->VMType->mVersion > MakeVersion(2, 0))
+	const PClassType *const parentVMType = parent->VMType;
+
+	if (parentVMType == nullptr)
+	{
+		// Abort when forward declared class is used as a parent. This edge case cannot be handled gracefully.
+		sc.Message(MSG_FATAL, "Tried to define class '%s' without definition of parent class '%s'.", typeName.GetChars(), parent->TypeName.GetChars());
+	}
+	else if (parentVMType->mVersion > MakeVersion(2, 0))
 	{
 		sc.Message(MSG_ERROR, "Parent class %s of %s not accessible to DECORATE", parent->TypeName.GetChars(), typeName.GetChars());
 	}