RoleValue for Root member

[en1]

Hi!
I need formula like this : roleValue('Day_last',currentmember([Data Item1]))
where [Data Item1] some hierarchy

'Day_last' - custom role value

in this case - error: The query contains a role value function with an invalid first argument

but if [Data Item1] is set members from this hierarchy except root member formula roleValue('Day_last',[Data Item1]) works.

It seems problem in use roleValue function for root member?