diff --git a/src/dobject.h b/src/dobject.h
index d043dfada..5cf877fed 100644
--- a/src/dobject.h
+++ b/src/dobject.h
@@ -198,7 +198,7 @@ protected:
 	enum { MetaClassNum = CLASSREG_PClass };
 
 	// Per-instance variables. There are four.
-#ifdef _DEBUG
+#ifndef NDEBUG
 public:
 	enum
 	{
diff --git a/src/scripting/vm/vm.h b/src/scripting/vm/vm.h
index 765ba4555..e5f7d1357 100644
--- a/src/scripting/vm/vm.h
+++ b/src/scripting/vm/vm.h
@@ -376,7 +376,7 @@ int VMCall(VMFunction *func, VMValue *params, int numparams, VMReturn *results,
 // variable name <x> at position <p>
 void NullParam(const char *varname);
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 bool AssertObject(void * ob);
 #endif
 
diff --git a/src/scripting/vm/vmexec.cpp b/src/scripting/vm/vmexec.cpp
index cc88a0eaa..252371c40 100644
--- a/src/scripting/vm/vmexec.cpp
+++ b/src/scripting/vm/vmexec.cpp
@@ -229,7 +229,7 @@ void VMFillParams(VMValue *params, VMFrame *callee, int numparam)
 }
 
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 bool AssertObject(void * ob)
 {
 	auto obj = (DObject*)ob;