diff --git a/keystore/user_state.h b/keystore/user_state.h
index 2a52f81..902719c 100644
--- a/keystore/user_state.h
+++ b/keystore/user_state.h
@@ -29,7 +29,7 @@
 
 class UserState {
   public:
-    UserState(uid_t userId);
+    explicit UserState(uid_t userId);
     ~UserState();
 
     bool initialize();