diff --git a/src/or/relay.c b/src/or/relay.c
index f5e9a6bb8c..99750af975 100644
--- a/src/or/relay.c
+++ b/src/or/relay.c
@@ -615,6 +615,7 @@ relay_send_command_from_edge_(streamid_t stream_id, circuit_t *circ,
     cell.circ_id = circ->n_circ_id;
     cell_direction = CELL_DIRECTION_OUT;
   } else {
+    tor_assert(! cpath_layer);
     cell.circ_id = TO_OR_CIRCUIT(circ)->p_circ_id;
     cell_direction = CELL_DIRECTION_IN;
   }