Privileges Problem

I am having a problem with privileges related to a procedure.

According to documentation I read, as long as the procedure
has appropriate privileges to tables it uses, the USER / ROLE
only needs EXECUTE privileges on the procedure. It does not
seem to be working for me.

In case I can get an answer without posting all the gory details
of the real code, I will start with what I hope is an accurate but simplified model.

User: AUser
Role: CUST

Table A
AWINNER varchar(1)

Procedure PROC.

GRANTS.
grant select, update on A to Procedure PROC;
grant execute on procedure PROC to CUST;
grant role CUST to AUser;

Now, when I log on as AUser with Role CUST and try and run PROC:

select is_winner FROM PROC(86)

I get the following error:
no permission for read/select access to COLUMN AWINNER;

A check of the RDB$USER_PRIVILEGES table shows that PROC has Select and Update privileges on Table A.

Thanks,
Clive.