diff --git a/src/fsm.js b/src/fsm.js
index e0ac974..a81cdfa 100644
--- a/src/fsm.js
+++ b/src/fsm.js
@@ -47,7 +47,7 @@ export function openAutomaton(states, editable = false) {
    */
   function updateUIState() {
     wordInput.value = builder;
-    if (typeof Array.from(state).find((state) => states[state].accepting) === 'undefined') {
+    if (typeof Array.from(state).find((state) => states[state]?.accepting) === 'undefined') {
       light.classList.remove(IS_VALID);
       light.classList.add(IS_INVALID);
     } else {