diff --git a/lib/compress.js b/lib/compress.js
index 121e312a..13ae7c70 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2067,7 +2067,8 @@ merge(Compressor.prototype, {
                     && self.left.right instanceof AST_Constant
                     && self.right instanceof AST_Binary
                     && self.right.operator == "+"
-                    && self.right.left instanceof AST_Constant) {
+                    && self.right.left instanceof AST_Constant
+                    && self.right.is_string(compressor)) {
                     self = make_node(AST_Binary, self, {
                         operator: "+",
                         left: make_node(AST_Binary, self.left, {