diff --git a/lib/rrule.js b/lib/rrule.js
index b349f37e..c978e1d8 100644
--- a/lib/rrule.js
+++ b/lib/rrule.js
@@ -446,13 +446,16 @@
     // used by toString()
     this.origOptions = {}
 
+    this.options = {}
+
     var invalid = []
     var keys = Object.keys(options)
     var defaultKeys = Object.keys(RRule.DEFAULT_OPTIONS)
 
-    // Shallow copy for origOptions and check for invalid
+    // Shallow copy for options and origOptions and check for invalid
     keys.forEach(function (key) {
       this.origOptions[key] = options[key]
+      this.options[key] = options[key]
       if (!contains(defaultKeys, key)) invalid.push(key)
     }, this)
 
@@ -464,10 +467,10 @@
 
     // Merge in default options
     defaultKeys.forEach(function (key) {
-      if (!contains(keys, key)) options[key] = RRule.DEFAULT_OPTIONS[key]
-    })
+      if (!contains(keys, key)) this.options[key] = RRule.DEFAULT_OPTIONS[key]
+    }, this)
 
-    var opts = this.options = options
+    var opts = this.options
 
     if (opts.byeaster !== null) opts.freq = RRule.YEARLY
     if (!opts.dtstart) opts.dtstart = new Date(new Date().setMilliseconds(0))
diff --git a/test/rrule.js b/test/rrule.js
index e6d1675a..7ca4e794 100644
--- a/test/rrule.js
+++ b/test/rrule.js
@@ -88,6 +88,24 @@ describe('RRule', function () {
     })
   })
 
+  it('does not mutate the passed-in options object', function () {
+    var options = {
+      freq: RRule.MONTHLY,
+      dtstart: new Date(2013, 0, 1),
+      count: 3,
+      bymonthday: [28]
+    }
+    var rule = new RRule(options)
+
+    assert.deepEqual(options, {
+      freq: RRule.MONTHLY,
+      dtstart: new Date(2013, 0, 1),
+      count: 3,
+      bymonthday: [28]
+    })
+    assert.deepEqual(rule.origOptions, options)
+  })
+
   testRecurring('missing Feb 28 https://github.com/jakubroztocil/rrule/issues/21',
     new RRule({
       freq: RRule.MONTHLY,