diff --git a/runtime/base/hash_set.h b/runtime/base/hash_set.h
index 12d3be7..f24a862 100644
--- a/runtime/base/hash_set.h
+++ b/runtime/base/hash_set.h
@@ -296,7 +296,7 @@
     return const_iterator(this, NumBuckets());
   }
 
-  bool Empty() {
+  bool Empty() const {
     return Size() == 0;
   }