in mangle_names there is a check that the variable name is legal and that it is not a reserved word. This should apply to propsmangle as well.

lib/propmangle.js

@@ -149,6 +149,7 @@ function mangle_properties(ast, options) {
     // only function declarations after this line
 
     function can_mangle(name) {
+        if (!is_identifier(name)) return false;
         if (unmangleable.indexOf(name) >= 0) return false;
         if (reserved.indexOf(name) >= 0) return false;
         if (options.only_cache) {