diff --git a/src/hb-iter.hh b/src/hb-iter.hh
index ebedc8483..8e4a8a906 100644
--- a/src/hb-iter.hh
+++ b/src/hb-iter.hh
@@ -60,7 +60,7 @@ struct hb_iter_t
   operator iter_t () { return iter(); }
   explicit_operator bool () const { return more (); }
   item_t& operator * () const { return item (); }
-  item_t& operator [] (unsigned i) const { return item_at (i); }
+  item_t& operator [] (signed i) const { return item_at ((unsigned) i); }
   iter_t& operator += (unsigned count) { forward (count); return *thiz(); }
   iter_t& operator ++ () { next (); return *thiz(); }
   iter_t& operator -= (unsigned count) { rewind (count); return *thiz(); }