The driver will exit without writing data or reporting an error:

   max_wire_version = mongoc_cluster_node_max_wire_version (&client->cluster, hint);
   if (max_wire_version == -1) {
      EXIT;
   }

The checked call to _mongoc_client_preselect, above, should ensure max_wire_version is non-negative, so this should be an ALWAYS_ASSERT instead of a regular check?