version_date is blank - mobile buy SDK android 2.1.0

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I am maintaining an android app that uses SDK v2.1.0 and I bumped into this problem. I tried to create a field for version_date inside Checkout.class and still I received this error
{"errors":{"version_date":[{"code":"blank","message":"can't be blank","options":{}}]}}

Can you please help me about this issue? I'm stuck in here and I can't get a checkout token because of this issue. Hoping for your soonest response. Cheers