diff --git a/src/hb-open-file.hh b/src/hb-open-file.hh
index de22ed5..32a223c 100644
--- a/src/hb-open-file.hh
+++ b/src/hb-open-file.hh
@@ -154,7 +154,8 @@
       c->align (4);
       const char *end = (const char *) c->head;
 
-      if (items[i].tag == HB_OT_TAG_head && end - start >= head::static_size)
+      if (items[i].tag == HB_OT_TAG_head &&
+	  (unsigned) (end - start) >= head::static_size)
       {
 	head *h = (head *) start;
 	checksum_adjustment = &h->checkSumAdjustment;