diff --git a/XI2.h b/XI2.h
index 6ba1377..3c39946 100644
--- a/XI2.h
+++ b/XI2.h
@@ -127,7 +127,7 @@
 #define XISetMask(ptr, event)   (((unsigned char*)(ptr))[(event)>>3] |=  (1 << ((event) & 7)))
 #define XIClearMask(ptr, event) (((unsigned char*)(ptr))[(event)>>3] &= ~(1 << ((event) & 7)))
 #define XIMaskIsSet(ptr, event) (((unsigned char*)(ptr))[(event)>>3] &   (1 << ((event) & 7)))
-#define XIMaskLen(event)        (((event + 7) >> 3))
+#define XIMaskLen(event)        (((event) >> 3) + 1)
 
 /* Fake device ID's for event selection */
 #define XIAllDevices                            0