diff --git a/dist/acorn.js b/dist/acorn.js
index 0523f0e3485d0100b6c83d9a290b6ea05bc1f921..1617d4aa489ae3fb014f3540e9b0df7a89a1944e 100644
--- a/dist/acorn.js
+++ b/dist/acorn.js
@@ -1835,7 +1835,7 @@
       if (checkClashes) {
         if (has(checkClashes, expr.name))
           { this.raiseRecoverable(expr.start, "Argument name clash"); }
-        checkClashes[expr.name] = true;
+          Object.defineProperty(checkClashes, expr.name, { value: true, writable: true, enumerable: true, configurable: true });
       }
       if (bindingType !== BIND_NONE && bindingType !== BIND_OUTSIDE) { this.declareName(expr.name, bindingType, expr.start); }
       break