Eval method behaving strangely

Hello. This is very strange but we have a pipeline that has pipeline parameters / arguments. 1 for month and the other year:

This works fine when the months is 7 or less (july). once you put the param = 08 for august and year 2020 this strange error occurs. we are using the eval() method and mapping eval(_month) to month variable and eval(_year) to year variable. this makes absolutely no sense!

Using 07 2020 there are no errors or any months prior to it:

What gives?