diff --git a/passphrase_cache.cc b/passphrase_cache.cc
index 1be70b7..0027f36 100644
--- a/passphrase_cache.cc
+++ b/passphrase_cache.cc
@@ -59,7 +59,7 @@ namespace pEp {
 
     bool PassphraseCache::for_each_passphrase(const passphrase_callee& callee)
     {
-        if (callee(""))
+        if (callee(std::string()))
             return true;
 
         {