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Purchase invoice error( expected str instance, NoneType found)

I need help with this error, i am trying to save a purchase invoice and submit it, but i am getting this error
TypeError: sequence item 2: expected str instance, NoneType found…

the purchase invoice contains items that are stock, non-stock, and asset items,

i tried deleting item 2 and i got the error item 1, deleted item 1 i got error item 0,

i even tried submitting the each of item category separately and i still got the error

can anybody help me please?!

Can you please provide your doctype’s field types?