diff --git a/wv-window.rkt b/wv-window.rkt
index 8eab99d..6e210a8 100644
--- a/wv-window.rkt
+++ b/wv-window.rkt
@@ -93,7 +93,7 @@
                                             (format
                                              (string-append
                                               "{ let el = document.getElementById('~a');\n"
-                                              "  let a = el.getAttribute('type');\n"
+                                              "  let a = (el === null) ? null : el.getAttribute('type');\n"
                                               "  if (a === null) {\n"
                                               "    return false;\n"
                                               "  } else {\n"