diff --git a/XI2.h b/XI2.h
index 06bc740..6323204 100644
--- a/XI2.h
+++ b/XI2.h
@@ -112,7 +112,7 @@
 #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 >> 3))
+#define XIMaskLen(event)        (((event + 7) >> 3))
 
 /* Fake device ID's for event selection */
 #define XIAllDevices                            0