I am working in a purchase order form with the following structure.

Purchase order:

date - date

company - text

provider - text

email - email

subform 

total order cost - currency

Subform (in purchase order):

reference - lookup to product catalog

description - text

cost - currency

IVA - number

cost plus IVA - currency 

quantity - number

final cost - currency

The form works perfect,  but on edit mode I am having some trouble. The problem is that when I want to edit the Cost field or the quantity field in the subform it tells me that there is an error, and the values doesn´t update anymore. Currently I can just delete the row and start again with it, its not a big big problem, but I want to avoid having to explain to every user what to do when they want to edit this fields. is there a way to fix this?

Any help will be very appreciated.