Change Length Of Field

I have a database on sql2000. I want to change the length of a field in Table A, but it is related to another field in Table B. When I tried to change the length of the field in Table A via

ALTER TABLE table ALTER COLUMN field varchar(5).The query analyzer generates the following error.

Server: Msg 5074, Level 16, State 8, Line 1
The object 'UQ__table__77BFCB91' is dependent on column 'field'.
Server: Msg 4922, Level 16, State 1, Line 1

ALTER TABLE ALTER COLUMN field failed because one or more objects access this column.While when I tried the same query on the field in TABLE B, the query completed without any errors.