Granting user defined role to dbo

Hi,

I have one login say test and executed:

use <db name>

go

sp_changedbowner test

I have user defined role role1 in this database.

I want to grant role1 to test. Now test has user dbo in the database. When I tried to grant this role to test/dbo there is error:

"Add member failed for databaserole 'role1'

.....

cannot user special principal 'dbo' (... Error 15405).

In SSMS, I right clicked login name, in properties selected user mapping, selected database, selected that role. When I clicked OK there is above error. Any help?

Thanks