diff --git a/lib/output.js b/lib/output.js
index 7f7e74df..68e3258a 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -482,6 +482,7 @@ function OutputStream(options) {
                     || parent instanceof AST_Sequence && parent.expressions[0] === node
                     || parent instanceof AST_Sub && parent.expression === node
                     || parent instanceof AST_UnaryPostfix) {
+                    if (!node.start) return;
                     var text = node.start.comments_before;
                     if (text && text._dumped !== self) {
                         text._dumped = self;