1 files changed, 3 insertions, 0 deletions
diff --git a/lib/js/urweb.js b/lib/js/urweb.js
index c7cecbbf..4daee66d 100644
--- a/lib/js/urweb.js
+++ b/lib/js/urweb.js
@@ -301,6 +301,9 @@ function listener() {
 }
 
 function rv(chn, parse, k) {
+  if (chn == null)
+    return;
+
   if (chn < 0)
     whine("Out-of-bounds channel receive");