diff --git a/src/backends/x11/meta-seat-x11.c b/src/backends/x11/meta-seat-x11.c
index 7ed00307f..6ae6f5f3b 100644
--- a/src/backends/x11/meta-seat-x11.c
+++ b/src/backends/x11/meta-seat-x11.c
@@ -1514,7 +1514,7 @@ translate_state (XIButtonState   *button_state,
 
   if (button_state)
     {
-      for (i = 1; i <= button_state->mask_len * 8; i++)
+      for (i = 1; i < button_state->mask_len * 8; i++)
         {
           if (!XIMaskIsSet (button_state->mask, i))
             continue;