diff --git a/src/dobjtype.cpp b/src/dobjtype.cpp
index d9af4044a..7e80eca0c 100644
--- a/src/dobjtype.cpp
+++ b/src/dobjtype.cpp
@@ -2322,12 +2322,19 @@ PClass *PClass::CreateDerivedClass(FName name, unsigned int size)
 	const PClass *existclass = FindClass(name);
 
 	// This is a placeholder so fill it in
-	if (existclass != NULL && existclass->Size == (unsigned)-1)
+	if (existclass != NULL && (existclass->Size == TClass_Fatal || existclass->Size == TClass_Nonfatal))
 	{
 		type = const_cast<PClass*>(existclass);
 		if (!IsDescendantOf(type->ParentClass))
 		{
-			I_Error("%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			if (existclass->Size == TClass_Fatal)
+			{
+				I_Error("%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			}
+			else
+			{
+				Printf(TEXTCOLOR_RED "%s must inherit from %s but doesn't.", name.GetChars(), type->ParentClass->TypeName.GetChars());
+			}
 		}
 		DPrintf("Defining placeholder class %s\n", name.GetChars());
 		notnew = true;
@@ -2380,7 +2387,7 @@ unsigned int PClass::Extend(unsigned int extension)
 //
 //==========================================================================
 
-PClass *PClass::FindClassTentative(FName name)
+PClass *PClass::FindClassTentative(FName name, bool fatal)
 {
 	if (name == NAME_None)
 	{
@@ -2400,7 +2407,7 @@ PClass *PClass::FindClassTentative(FName name)
 
 	type->TypeName = name;
 	type->ParentClass = this;
-	type->Size = -1;
+	type->Size = fatal? TClass_Fatal : TClass_Nonfatal;
 	type->bRuntimeClass = true;
 	TypeTable.AddType(type, RUNTIME_CLASS(PClass), (intptr_t)type->Outer, name, bucket);
 	return type;
diff --git a/src/dobjtype.h b/src/dobjtype.h
index 313130ddb..900059cab 100644
--- a/src/dobjtype.h
+++ b/src/dobjtype.h
@@ -599,6 +599,12 @@ public:
 
 // Meta-info for every class derived from DObject ---------------------------
 
+enum
+{
+	TClass_Fatal = UINT_MAX,
+	TClass_Nonfatal = UINT_MAX - 1
+};
+
 class PClassClass;
 class PClass : public PStruct
 {
@@ -661,7 +667,7 @@ public:
 	static PClassActor *FindActor(const FString &name)	{ return FindActor(FName(name, true)); }
 	static PClassActor *FindActor(ENamedName name)		{ return FindActor(FName(name)); }
 	static PClassActor *FindActor(FName name);
-	PClass *FindClassTentative(FName name);	// not static!
+	PClass *FindClassTentative(FName name, bool fatal = true);	// not static!
 
 	static TArray<PClass *> AllClasses;
 
diff --git a/src/thingdef/thingdef.cpp b/src/thingdef/thingdef.cpp
index 2d179e01d..9f0a9a951 100644
--- a/src/thingdef/thingdef.cpp
+++ b/src/thingdef/thingdef.cpp
@@ -340,10 +340,15 @@ static void FinishThingdef()
 	{
 		PClassActor *ti = PClassActor::AllActorClasses[i];
 
-		if (ti->Size == (unsigned)-1)
+		if (ti->Size == TClass_Fatal || ti->Size == TClass_Nonfatal)
 		{
-			Printf("Class %s referenced but not defined\n", ti->TypeName.GetChars());
-			errorcount++;
+			Printf(TEXTCOLOR_RED "Class %s referenced but not defined\n", ti->TypeName.GetChars());
+			if (ti->Size == TClass_Fatal) errorcount++;
+			else
+			{
+				// In order to prevent a crash, this class needs to be completed, even though it defines nothing.
+				ti->ParentClass->CreateDerivedClass(ti->TypeName, ti->ParentClass->Size);
+			}
 			continue;
 		}
 
diff --git a/src/thingdef/thingdef_expression.cpp b/src/thingdef/thingdef_expression.cpp
index 9124071e5..b86c690f5 100644
--- a/src/thingdef/thingdef_expression.cpp
+++ b/src/thingdef/thingdef_expression.cpp
@@ -3546,23 +3546,13 @@ FxExpression *FxClassTypeCast::Resolve(FCompileContext &ctx)
 
 		if (clsname != NAME_None)
 		{
-			cls = desttype->FindClassTentative(clsname);
-			if (cls == NULL)
+			// for backwards compatibility with old times it cannot be made a fatal error, if the class is not defined. :(
+			cls = desttype->FindClassTentative(clsname, false);
+			if (!cls->IsDescendantOf(desttype))
 			{
-				/* lax */
-				// Since this happens in released WADs it must pass without a terminal error... :(
-				ScriptPosition.Message(MSG_WARNING,
-					"Unknown class name '%s'", 
-					clsname.GetChars(), desttype->TypeName.GetChars());
-			}
-			else 
-			{
-				if (!cls->IsDescendantOf(desttype))
-				{
-					ScriptPosition.Message(MSG_ERROR,"class '%s' is not compatible with '%s'", clsname.GetChars(), desttype->TypeName.GetChars());
-					delete this;
-					return NULL;
-				}
+				ScriptPosition.Message(MSG_ERROR,"class '%s' is not compatible with '%s'", clsname.GetChars(), desttype->TypeName.GetChars());
+				delete this;
+				return NULL;
 			}
 			ScriptPosition.Message(MSG_DEBUG,"resolving '%s' as class name", clsname.GetChars());
 		}