diff --git a/acorn.js b/acorn.js
index 8c9053be95..c95d6d00db 100644
--- a/acorn.js
+++ b/acorn.js
@@ -1550,7 +1550,7 @@
     if (tokType.isAssign) {
       var node = startNodeFrom(left);
       node.operator = tokVal;
-      node.left = left;
+      node.left = toAssignable(left);
       next();
       node.right = parseMaybeAssign(noIn);
       checkLVal(left);