diff --git a/src/calls-contacts-provider.c b/src/calls-contacts-provider.c
index 0d5f95a..41faeb2 100644
--- a/src/calls-contacts-provider.c
+++ b/src/calls-contacts-provider.c
@@ -441,6 +441,7 @@ calls_contacts_provider_lookup_id (CallsContactsProvider *self,
   g_autoptr (CallsBestMatch) best_match = NULL;
 
   g_return_val_if_fail (CALLS_IS_CONTACTS_PROVIDER (self), NULL);
+  g_return_val_if_fail (id, NULL);
 
   best_match = g_hash_table_lookup (self->best_matches, id);