diff --git a/lib/propmangle.js b/lib/propmangle.js
index efb31cc1..f17909d7 100644
--- a/lib/propmangle.js
+++ b/lib/propmangle.js
@@ -78,7 +78,8 @@ function mangle_properties(ast, options) {
         reserved: null,
     });
 
-    var reserved = options.reserved || [];
+    var reserved = options.reserved;
+    if (!Array.isArray(reserved)) reserved = [];
     if (!options.builtins) find_builtins(reserved);
 
     var cache = options.cache;
diff --git a/lib/scope.js b/lib/scope.js
index 8bc96072..ea43f752 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -377,13 +377,15 @@ AST_Symbol.DEFMETHOD("global", function(){
 });
 
 AST_Toplevel.DEFMETHOD("_default_mangler_options", function(options){
-    return defaults(options, {
+    options = defaults(options, {
         eval        : false,
         ie8         : false,
         keep_fnames : false,
         reserved    : [],
         toplevel    : false,
     });
+    if (!Array.isArray(options.reserved)) options.reserved = [];
+    return options;
 });
 
 AST_Toplevel.DEFMETHOD("mangle_names", function(options){