diff --git a/ai/ai_event.h b/ai/ai_event.h
index 7f96f71b85..1f49f338bd 100644
--- a/ai/ai_event.h
+++ b/ai/ai_event.h
@@ -20,9 +20,9 @@
  *  Ugly, I know, but it works! */
 
 #	ifdef DEF_EVENTS
-	void empty_function(PlayerID player, int event, ...) {}
+	void CDECL empty_function(PlayerID player, int event, ...) {}
 #	else
-	extern void empty_function(PlayerID player, int event, ...);
+	extern void CDECL empty_function(PlayerID player, int event, ...);
 #	endif
 #	define ai_event empty_function
 
diff --git a/saveload.c b/saveload.c
index dc7485eacb..8cfc878bbd 100644
--- a/saveload.c
+++ b/saveload.c
@@ -953,15 +953,8 @@ static void WriteMem(uint size)
 //********************************************
 
 #if defined(WITH_ZLIB)
-
-// This is needed to zlib uses the stdcall calling convention on visual studio
-#ifdef _MSC_VER
-#	ifndef ZLIB_WINAPI
-#		define ZLIB_WINAPI
-#	endif
-#endif
-
 #include <zlib.h>
+
 static z_stream _z;
 
 static bool InitReadZlib(void)
diff --git a/stdafx.h b/stdafx.h
index 74c44e46de..b42b3688fb 100644
--- a/stdafx.h
+++ b/stdafx.h
@@ -142,6 +142,14 @@
 # undef TTD_ALIGNMENT_4
 # undef TTD_ALIGNMENT_2
 # define GCC_PACK
+
+// This is needed to zlib uses the stdcall calling convention on visual studio, also used with libpng (VS6 warning)
+# if defined(WITH_ZLIB) || defined(WITH_PNG)
+#  ifndef ZLIB_WINAPI
+#   define ZLIB_WINAPI
+#  endif
+# endif
+
 #endif /* defined(_MSC_VER) */