Can't Change BillingID in Invoice

[Mark Phillips]

I was experimenting with using the business features of gnucash 2.4.13 and
I did the following

1. Created a vendor
2. Created a bill, but did NOT enter the billingID
3 Created a job for this bill
4. Posted the bill (invoice) to a/p.

I then realized that I had missed adding in the billingId, so I tried to go
back to the bill and edit it. The cursor is active in the billingId field
when I open the bill, but I cannot type anything in that field.

I tried
1. uposting the bill, and still cannot edit the billingId field
2. making the bill inactive, and still cannot edit the billingid field.

Is there some reason I cannot edit the billingID field?

Thanks,

Mark