Credit Memo with null distribution amount

We have a credit memo invoice that created multiple distributions when matched.  In  the ap_invoice_distributions_all table there are 22 records, but in the forms only 6 distributions are visible.  One of the visible records causes an error to be received (FRM-40202: Field must be entered) and has a null amount.  The amount field is immediately set to read-only and cannot be updated.  I am assuming that the null amount is causing the other distributions not to be displayed. 

I need to know a) what caused the null amount, and b) how do I fix it?  Thanks.