Take each first element of the group by

Question

How can I take each first element in the group with EF 5 ?

      var result = await context.SomeDbSet
          .Where(...)
          .GroupBy(x => new { x.SomeField, ... })
          .Select(x => x.First())
          .ToListAsync();

I am getting not supported exception.

How to correctly rewrite query? Thanks.