unable to removeInteration in modify-features example

![image](https://f.cloud.github.com/assets/6138648/1703433/912bc8de-60b5-11e3-89d7-1f035cbcb4b2.png)

I type this in console
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map.removeInteraction(modify)
console error log
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TypeError: Cannot read property 'length' of undefined
Uncaught AssertionError: Assertion failed: instanceof check failed. asserts.js:102
Uncaught AssertionError: Assertion failed: instanceof check failed. asserts.js:102
Uncaught AssertionError: Assertion failed: instanceof check failed. asserts.js:102
GET http://ol3js.org/en/master/css/ol.css 404 (Not Found) modify-features.html:7

And the modify handler is still active.