diff --git a/headers/os/interface/InterfaceDefs.h b/headers/os/interface/InterfaceDefs.h
index 06c35b6842..bf9c3f184c 100644
--- a/headers/os/interface/InterfaceDefs.h
+++ b/headers/os/interface/InterfaceDefs.h
@@ -61,7 +61,11 @@ struct key_info {
 #define B_UTF8_TRADEMARK	"\xE2\x84\xA2"
 #define B_UTF8_SMILING_FACE	"\xE2\x98\xBB"
 #define B_UTF8_HIROSHI		"\xE5\xBC\x98"
-#define B_MAX_MOUSE_BUTTONS 16
+#ifdef COMPILE_FOR_R5
+	#define B_MAX_MOUSE_BUTTONS 3
+#else
+	#define B_MAX_MOUSE_BUTTONS 16
+#endif
 
 /*----------------------------------------------------------------*/