Interstitial ads crash game when close

Hi Guys!!
I created a game with cocos creator, and integrated Admob on it using SDKBOX.
well, everything works just fine, it shows the ad, but when i click on the “x” icon to close it or press the back button on android, the game crashes.
I followed those tutorials…

http://docs.sdkbox.com/en/qa/cocos_creator/

http://docs.sdkbox.com/en/qa/integration-admob-to-creator/

Some Aditional information:
Building for Android
Using android 20
Compiling using Cocos Creator own compile option

I will update the google play services.
Thanks.