diff --git a/src/libutil/lru-cache.hh b/src/libutil/lru-cache.hh
index 0e19517ed..6e14cac35 100644
--- a/src/libutil/lru-cache.hh
+++ b/src/libutil/lru-cache.hh
@@ -89,7 +89,7 @@ public:
         return i->second.second;
     }
 
-    size_t size()
+    size_t size() const
     {
         return data.size();
     }