diff --git a/src/dobjtype.cpp b/src/dobjtype.cpp
index 76330867e..313ee7389 100644
--- a/src/dobjtype.cpp
+++ b/src/dobjtype.cpp
@@ -3194,34 +3194,37 @@ void PClass::Derive(PClass *newclass, FName name)
 
 void PClass::InitializeDefaults()
 {
-	assert(Defaults == NULL);
-	Defaults = (BYTE *)M_Malloc(Size);
-
-	// run the constructor on the defaults to set the vtbl pointer which is needed to run class-aware functions on them.
-	// Temporarily setting bSerialOverride prevents linking into the thinker chains.
-	auto s = DThinker::bSerialOverride;
-	DThinker::bSerialOverride = true;
-	ConstructNative(Defaults);
-	DThinker::bSerialOverride = s;
-	// We must unlink the defaults from the class list because it's just a static block of data to the engine.
-	DObject *optr = (DObject*)Defaults;
-	GC::Root = optr->ObjNext;
-	optr->ObjNext = nullptr;
-	optr->SetClass(this);
-
-
-	// Copy the defaults from the parent but leave the DObject part alone because it contains important data.
-	if (ParentClass->Defaults != NULL)
+	if (IsKindOf(RUNTIME_CLASS(PClassActor)))
 	{
-		memcpy(Defaults + sizeof(DObject), ParentClass->Defaults + sizeof(DObject), ParentClass->Size - sizeof(DObject));
-		if (Size > ParentClass->Size)
+		assert(Defaults == NULL);
+		Defaults = (BYTE *)M_Malloc(Size);
+
+		// run the constructor on the defaults to set the vtbl pointer which is needed to run class-aware functions on them.
+		// Temporarily setting bSerialOverride prevents linking into the thinker chains.
+		auto s = DThinker::bSerialOverride;
+		DThinker::bSerialOverride = true;
+		ConstructNative(Defaults);
+		DThinker::bSerialOverride = s;
+		// We must unlink the defaults from the class list because it's just a static block of data to the engine.
+		DObject *optr = (DObject*)Defaults;
+		GC::Root = optr->ObjNext;
+		optr->ObjNext = nullptr;
+		optr->SetClass(this);
+
+
+		// Copy the defaults from the parent but leave the DObject part alone because it contains important data.
+		if (ParentClass->Defaults != NULL)
 		{
-			memset(Defaults + ParentClass->Size, 0, Size - ParentClass->Size);
+			memcpy(Defaults + sizeof(DObject), ParentClass->Defaults + sizeof(DObject), ParentClass->Size - sizeof(DObject));
+			if (Size > ParentClass->Size)
+			{
+				memset(Defaults + ParentClass->Size, 0, Size - ParentClass->Size);
+			}
+		}
+		else
+		{
+			memset(Defaults + sizeof(DObject), 0, Size - sizeof(DObject));
 		}
-	}
-	else
-	{
-		memset(Defaults + sizeof(DObject), 0, Size - sizeof(DObject));
 	}
 
 	if (bRuntimeClass)
diff --git a/src/dthinker.cpp b/src/dthinker.cpp
index ac0abf3c5..7b63d2635 100644
--- a/src/dthinker.cpp
+++ b/src/dthinker.cpp
@@ -710,8 +710,12 @@ class DThinkerIterator : public DObject, public FThinkerIterator
 {
 	DECLARE_CLASS(DThinkerIterator, DObject)
 
+	DThinkerIterator()
+	{
+	}
+
 public:
-	DThinkerIterator(PClass *cls = nullptr, int statnum = MAX_STATNUM + 1)
+	DThinkerIterator(PClass *cls, int statnum = MAX_STATNUM + 1)
 		: FThinkerIterator(cls, statnum)
 	{
 	}
diff --git a/src/dthinker.h b/src/dthinker.h
index fce3e4f7a..e0dcb607f 100644
--- a/src/dthinker.h
+++ b/src/dthinker.h
@@ -125,6 +125,9 @@ public:
 	FThinkerIterator (const PClass *type, int statnum, DThinker *prev);
 	DThinker *Next (bool exact = false);
 	void Reinit ();
+
+protected:
+	FThinkerIterator() {}
 };
 
 template <class T> class TThinkerIterator : public FThinkerIterator
diff --git a/src/p_maputl.cpp b/src/p_maputl.cpp
index ea2434b25..5771461bb 100644
--- a/src/p_maputl.cpp
+++ b/src/p_maputl.cpp
@@ -1177,6 +1177,11 @@ class DBlockThingsIterator : public DObject, public FMultiBlockThingsIterator
 {
 	DECLARE_CLASS(DBlockThingsIterator, DObject);
 	FPortalGroupArray check;
+protected:
+	DBlockThingsIterator()
+		:FMultiBlockThingsIterator(check)
+	{
+	}
 public:
 	FMultiBlockThingsIterator::CheckResult cres;
 
@@ -1186,7 +1191,7 @@ public:
 		return FMultiBlockThingsIterator::Next(&cres);
 	}
 
-	DBlockThingsIterator(AActor *origin = nullptr, double checkradius = -1, bool ignorerestricted = false)
+	DBlockThingsIterator(AActor *origin, double checkradius = -1, bool ignorerestricted = false)
 		: FMultiBlockThingsIterator(check, origin, checkradius, ignorerestricted)
 	{
 		cres.thing = nullptr;
diff --git a/src/p_maputl.h b/src/p_maputl.h
index 81d3c9d13..5e869c4e6 100644
--- a/src/p_maputl.h
+++ b/src/p_maputl.h
@@ -327,6 +327,8 @@ class FMultiBlockThingsIterator
 
 	void startIteratorForGroup(int group);
 
+protected:
+	FMultiBlockThingsIterator(FPortalGroupArray &check) : checklist(check) {}
 public:
 
 	struct CheckResult
diff --git a/src/scripting/zscript/zcc_compile.cpp b/src/scripting/zscript/zcc_compile.cpp
index 43f5b6774..610e7554d 100644
--- a/src/scripting/zscript/zcc_compile.cpp
+++ b/src/scripting/zscript/zcc_compile.cpp
@@ -1874,7 +1874,14 @@ void ZCCCompiler::InitDefaults()
 		if (!c->Type()->IsDescendantOf(RUNTIME_CLASS(AActor)))
 		{
 			if (c->Defaults.Size()) Error(c->cls, "%s: Non-actor classes may not have defaults", c->Type()->TypeName.GetChars());
-			if (c->Type()->ParentClass) c->Type()->ParentClass->DeriveData(c->Type());
+			if (c->Type()->ParentClass)
+			{
+				auto ti = static_cast<PClassActor *>(c->Type());
+				FString mename = ti->TypeName.GetChars();
+
+				ti->InitializeDefaults();
+				ti->ParentClass->DeriveData(ti);
+			}
 		}
 		else
 		{