diff --git a/src/policy_cache.hpp b/src/policy_cache.hpp
index 7d0a75c6..c8646a78 100644
--- a/src/policy_cache.hpp
+++ b/src/policy_cache.hpp
@@ -62,7 +62,7 @@ public:
 
   const
   nonstd::optional<std::string>
-  find(std::string const key_)
+  find(std::string const &key_)
   {
     nonstd::optional<std::string> rv;