diff --git a/lib/libc/gen/ttyname.c b/lib/libc/gen/ttyname.c
index 013796c4a78..06dd1b95fb7 100644
--- a/lib/libc/gen/ttyname.c
+++ b/lib/libc/gen/ttyname.c
@@ -124,7 +124,8 @@ ttyname(int fd)
 			return (NULL);
 		}
 	}
-	ttyname_r(fd, buf, sizeof(_PATH_DEV) + MAXNAMLEN);
+	if (ttyname_r(fd, buf, sizeof(_PATH_DEV) + MAXNAMLEN) != 0)
+		return (NULL);
 	return (buf);
 }