diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index ace2a10..749ef9b 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -499,12 +499,12 @@
 			unsigned int cluster_start,
 			unsigned int cluster_end)
 {
-  hb_mask_t not_mask = ~mask;
-  value &= mask;
-
   if (!mask)
     return;
 
+  hb_mask_t not_mask = ~mask;
+  value &= mask;
+
   unsigned int count = len;
   for (unsigned int i = 0; i < count; i++)
     if (cluster_start <= info[i].cluster && info[i].cluster < cluster_end)