Question : Removing NOT NULL to a column doesn't work

Problem with a Not Null column in Firebird 1.5.
I attach a small database to reproduce the following problem:
- In the table FACTURA, I remove the constraint NOT NULL from the column FACTIPOIMPOSITIVO1 (either by code, either by using IBExpert)
- I try to set the value of that column to NULL and I get the error 'validation error for column FACTIPOIMPOSITIVO1, value '*** null ***'
- I get the same error executing my application when inserting a new record.

I'm out of ideas and I need an urgent solution. Please help!
NOTE: after unzipping, the database should be renamed to *.fdb

NotNullTest.zip (90 KB) (File Type Details) 

Original database. Should be renamed to *.fdb

Answer : Removing NOT NULL to a column doesn't work

ensure you are the sole user

drop constraint

commit;

update table set field = NULL