diff --git a/src/cups-pk-helper-mechanism.c b/src/cups-pk-helper-mechanism.c
index 273c262..8b3d781 100644
--- a/src/cups-pk-helper-mechanism.c
+++ b/src/cups-pk-helper-mechanism.c
@@ -205,6 +205,8 @@ register_mechanism (CphMechanism *mechanism)
 {
         GError *error;
 
+        g_return_val_if_fail (CPH_IS_MECHANISM (mechanism), FALSE);
+
         mechanism->priv->pol_auth = polkit_authority_get ();
 
         error = NULL;
@@ -238,7 +240,8 @@ cph_mechanism_new (void)
         object = g_object_new (CPH_TYPE_MECHANISM, NULL);
 
         if (!register_mechanism (CPH_MECHANISM (object))) {
-                g_object_unref (object);
+                if (object)
+                        g_object_unref (object);
                 return NULL;
         }