Payments on account of Class 2 NI for 2015/2016 within self-assessment

Are payments on account due in respect of class 2 NI for self-assessment, as they are for class 4 NI, or should it be treated like student loan repayments and excluded from calculation of payments on account?
I’m calculating the self-assessment payments due for 2015/2016, in the context of making a claim to reduce payments on account where the only other liability is less than £100 in class 4 NI.
I've asked HMRC and got a 'don't know' response.

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