diff --git a/lib/minify.js b/lib/minify.js
index 9eebc373..a99c58b9 100644
--- a/lib/minify.js
+++ b/lib/minify.js
@@ -87,6 +87,8 @@ function minify(files, options) {
                 properties: false,
                 reserved: [],
                 toplevel: false,
+                leading_character_set : "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_",
+                trailing_character_set: "0123456789",
             }, true);
             if (options.mangle.properties) {
                 if (typeof options.mangle.properties != "object") {