diff --git a/src/proto_sockpair.c b/src/proto_sockpair.c
index a4faa37..e7dd670 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -80,6 +80,9 @@
 /* Add <listener> to the list of sockpair listeners (port is ignored). The
  * listener's state is automatically updated from LI_INIT to LI_ASSIGNED.
  * The number of listeners for the protocol is updated.
+ *
+ * Must be called with proto_lock held.
+ *
  */
 static void sockpair_add_listener(struct listener *listener, int port)
 {
@@ -97,6 +100,8 @@
  * loose them across the fork(). A call to uxst_enable_listeners() is needed
  * to complete initialization.
  *
+ * Must be called with proto_lock held.
+ *
  * The return value is composed from ERR_NONE, ERR_RETRYABLE and ERR_FATAL.
  */
 static int sockpair_bind_listeners(struct protocol *proto, char *errmsg, int errlen)