diff --git a/common/hashmap.h b/common/hashmap.h
index b8df274f09d..842296db991 100644
--- a/common/hashmap.h
+++ b/common/hashmap.h
@@ -172,7 +172,7 @@ public:
 
 	size_t erase(const Key &key);
 
-	uint size() { return _nele; }
+	uint size() const { return _nele; }
 
 	const_iterator	begin() const {
 		// Find and return the first non-empty entry