update multiple rows with single update SQL Server

Why does this SQL query not work :

update MyTable
set IsOk = 1
where MyName = 'OK'

error is :

Subquery retu开发者_开发百科rned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

thanks for help

Can you check 'OK' value might be containing dirty value? If not, 1 value might be also.