diff --git a/lib/compress.js b/lib/compress.js
index 7c4c910d..d78d41d6 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1744,7 +1744,7 @@ merge(Compressor.prototype, {
         }
         if (self.operator == "+" && self.right instanceof AST_String
             && self.right.getValue() === "" && self.left instanceof AST_Binary
-            && self.left.operator == "+" && self.left.is_string()) {
+            && self.left.operator == "+" && self.left.is_string(compressor)) {
             return self.left;
         }
         return self;