diff --git a/idlib/Lib.h b/idlib/Lib.h
index 90287ba..58d5ac3 100644
--- a/idlib/Lib.h
+++ b/idlib/Lib.h
@@ -122,7 +122,7 @@ void	SixtetsForInt( byte *out, int src);
 int		IntForSixtets( byte *in );
 
 
-#ifdef _DEBUG
+#if defined(_DEBUG) && !defined(NDEBUG)
 void AssertFailed( const char *file, int line, const char *expression );
 #undef assert
 #define assert( X )		if ( X ) { } else AssertFailed( __FILE__, __LINE__, #X )