diff --git a/lib/DoctorKarma/DAO/User.pm b/lib/DoctorKarma/DAO/User.pm
index e09d9ce..bce012d 100644
--- a/lib/DoctorKarma/DAO/User.pm
+++ b/lib/DoctorKarma/DAO/User.pm
@@ -153,7 +153,7 @@ EOF
         $user->karma( $user_with_new_karma->karma );
         $logger->log_info( 'User '
               . $user->first_name . ':'
-              . $user->username . ':'
+              . ($user->username // '') . ':'
               . $user->id_user
               . ' has now '
               . $user->karma
diff --git a/lib/DoctorKarma/Model/User.pm b/lib/DoctorKarma/Model/User.pm
index 713e2db..44284b0 100644
--- a/lib/DoctorKarma/Model/User.pm
+++ b/lib/DoctorKarma/Model/User.pm
@@ -33,7 +33,7 @@ sub first_name {
     if (exists $self->{first_name}) {
         return $self->{first_name};
     }
-    return;
+    return undef;
 }
 
 sub id_user {
@@ -54,7 +54,7 @@ sub id_user {
         if ( exists $self->{username} ) {
             return $self->{username};
         }
-        return;
+        return undef;
     }
 }