diff --git a/lib/minify.js b/lib/minify.js
index a68cbf3a..7ddfbeff 100644
--- a/lib/minify.js
+++ b/lib/minify.js
@@ -120,7 +120,7 @@ function minify(files, options) {
             if (typeof files == "string") {
                 files = [ files ];
             }
-            options.parse = options.parse || {};
+            options.parse = assign({}, options.parse);
             options.parse.toplevel = null;
             for (var name in files) if (HOP(files, name)) {
                 options.parse.filename = name;
@@ -159,6 +159,7 @@ function minify(files, options) {
             toplevel = mangle_properties(toplevel, options.mangle.properties);
         }
         if (timings) timings.output = Date.now();
+        options.output = assign({}, options.output);
         var result = {};
         if (options.output.ast) {
             result.ast = toplevel;
diff --git a/lib/scope.js b/lib/scope.js
index 6c883c66..e13e7f4b 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -405,7 +405,7 @@ AST_Toplevel.DEFMETHOD("_default_mangler_options", function(options) {
         reserved    : [],
         toplevel    : false,
     });
-    if (!Array.isArray(options.reserved)) options.reserved = [];
+    options.reserved = Array.isArray(options.reserved) ? options.reserved.slice() : [];
     // Never mangle arguments
     push_uniq(options.reserved, "arguments");
     return options;
diff --git a/lib/utils.js b/lib/utils.js
index 9121fa93..03e2a743 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -95,15 +95,14 @@ DefaultsError.croak = function(msg, defs) {
 };
 
 function defaults(args, defs, croak) {
-    if (args === true)
-        args = {};
-    var ret = args || {};
-    if (croak) for (var i in ret) if (HOP(ret, i) && !HOP(defs, i))
-        DefaultsError.croak("`" + i + "` is not a supported option", defs);
-    for (var i in defs) if (HOP(defs, i)) {
-        ret[i] = (args && HOP(args, i)) ? args[i] : defs[i];
+    if (args === true || !args) return defs;
+
+    for (var i in args) if (HOP(args, i)) {
+        if (croak && !HOP(defs, i))
+            DefaultsError.croak("`" + i + "` is not a supported option", defs);
+        defs[i] = args[i];
     }
-    return ret;
+    return defs;
 };
 
 function merge(obj, ext) {
@@ -115,6 +114,11 @@ function merge(obj, ext) {
     return count;
 };
 
+function assign(obj, ext) {
+    merge(obj, ext);
+    return obj;
+};
+
 function noop() {}
 function return_false() { return false; }
 function return_true() { return true; }
diff --git a/test/mocha/minify.js b/test/mocha/minify.js
index 65392ee4..01e0eb46 100644
--- a/test/mocha/minify.js
+++ b/test/mocha/minify.js
@@ -122,6 +122,32 @@ describe("minify", function() {
         assert.strictEqual(Uglify.minify("function this(){}").error.message, "Unexpected token: name (this)");
     });
 
+    it("Should not mutate options", function() {
+        [
+            {},
+            {
+                parse: {},
+                compress: {
+                    global_defs: {},
+                },
+                mangle: {
+                    reserved: [],
+                    // expect `cache` to be mutated
+                },
+                output: {},
+                rename: {
+                    reserved: [],
+                },
+                sourceMap: {},
+                // expect `nameCache` to be mutated
+            },
+        ].forEach(function(options) {
+            var optsString = JSON.stringify(options);
+            Uglify.minify("", options);
+            assert.strictEqual(JSON.stringify(options), optsString);
+        });
+    });
+
     describe("keep_quoted_props", function() {
         it("Should preserve quotes in object literals", function() {
             var js = 'var foo = {"x": 1, y: 2, \'z\': 3};';