Here is a small piece of code to back up my assertion that this is a datatype problem:

if object_id('tempdb..#ex', 'U') is not null

drop table #ex

create table #ex

(

someDate datetime null

)

insert #ex

(someDate)

select '20130101'

select isnull(someDate, 'nothing')

from #ex

group by isnull(someDate, 'nothing')

Msg 241, Level 16, State 1, Line 13

Conversion failed when converting date and/or time from character string.