diff --git a/acorn.js b/acorn.js
index 3f45779187..1e8fc48b37 100644
--- a/acorn.js
+++ b/acorn.js
@@ -2473,7 +2473,7 @@
     var expr = parseExprOps(noIn, noLess, isStatement);
     if (eat(_question)) {
       var node = startNodeAt(start);
-      if (eat(_eq)) {
+      if (options.playground && eat(_eq)) {
         var left = node.left = toAssignable(expr);
         if (left.type !== "MemberExpression") raise(left.start, "You can only use member expressions in memoization assignment");
         node.right = parseMaybeAssign(noIn);
diff --git a/package.json b/package.json
index 419331c645..1f3cf62207 100644
--- a/package.json
+++ b/package.json
@@ -2,7 +2,7 @@
   "name": "acorn-6to5",
   "description": "Acorn fork used by 6to5",
   "main": "acorn_csp.js",
-  "version": "0.11.1-14",
+  "version": "0.11.1-15",
   "maintainers": [
     {
       "name": "Marijn Haverbeke",