diff --git a/lib/compress.js b/lib/compress.js
index e2f2d6b4..a2f8f267 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -89,7 +89,7 @@ function Compressor(options, false_by_default) {
     }, true);
     var global_defs = this.options["global_defs"];
     if (typeof global_defs == "object") for (var key in global_defs) {
-        if (/^@/.test(key)) {
+        if (/^@/.test(key) && HOP(global_defs, key)) {
             var ast = parse(global_defs[key]);
             if (ast.body.length == 1 && ast.body[0] instanceof AST_SimpleStatement) {
                 global_defs[key.slice(1)] = ast.body[0].body;