diff --git a/SheepShaver/src/MacOSX/prefs_macosx.mm b/SheepShaver/src/MacOSX/prefs_macosx.mm
index 779ad629..7bda2b53 100644
--- a/SheepShaver/src/MacOSX/prefs_macosx.mm
+++ b/SheepShaver/src/MacOSX/prefs_macosx.mm
@@ -24,6 +24,9 @@
 
 #include "sysdeps.h"
 
+// The _UINT64 define is needed to guard against a typedef mismatch with Snow Leopard headers.
+#define _UINT64
+
 #include <Cocoa/Cocoa.h>
 #include "VMSettingsController.h"