So, I'm trying get the value of a field as a string, and all I get when I cast the self.pool.get method to a string is a big fat "None"

Here's my code:

ccNum = str( self.pool.get('account.voucher').browse(cr,uid,'card_number',context=None)[0] )

Can I not cast this to a string? If not, then what do I need to do to get the value out as a string instead of an object?