diff --git a/src/shared/ptyfwd.c b/src/shared/ptyfwd.c
index d9ed74b950..998ce96b54 100644
--- a/src/shared/ptyfwd.c
+++ b/src/shared/ptyfwd.c
@@ -763,6 +763,9 @@ int pty_forward_new(
         struct winsize ws;
         int r;
 
+        assert(master >= 0);
+        assert(ret);
+
         f = new(PTYForward, 1);
         if (!f)
                 return -ENOMEM;