diff --git a/src/or/connection.c b/src/or/connection.c
index 1e1320cea7..ecdb4dbeb3 100644
--- a/src/or/connection.c
+++ b/src/or/connection.c
@@ -2089,6 +2089,7 @@ assert_connection_ok(connection_t *conn, time_t now)
     case CONN_TYPE_OR:
       tor_assert(conn->state >= _OR_CONN_STATE_MIN);
       tor_assert(conn->state <= _OR_CONN_STATE_MAX);
+      tor_assert(conn->n_circuits >= 0);
       break;
     case CONN_TYPE_EXIT:
       tor_assert(conn->state >= _EXIT_CONN_STATE_MIN);