How to use distinct with group by in Linq to SQL

Programmer Question

I'm trying to convert the following sql to Linq 2 SQL:

select groupId, count(distinct(userId)) from processroundissueinstance 
group by groupId

Here is my code:

var q = from i in ProcessRoundIssueInstance
    group i by i.GroupID into g
    select new
    {
        Key = g.Key,
        Count = g.Select(x => x.UserID).Distinct().Count()
    };

When I run the code, I keep getting Invalid GroupID.
Any ideas? Seems the distinct is screwing things up..

Here is the generated sql:

SELECT [t1].[GroupID] AS [Key], (
SELECT COUNT(*)
FROM (
    SELECT DISTINCT [t2].[UserID]
    FROM [ProcessRoundIssueInstance] AS [t2]
    WHERE (([t1].[GroupID] IS NULL) AND ([t2].[GroupID] IS NULL)) 
       OR (([t1].[GroupID] IS NOT NULL) 
            AND ([t2].[GroupID] IS NOT NULL) 
            AND ([t1].[GroupID] = [t2].[GroupID]))
    ) AS [t3]
) AS [Count]
FROM (
    SELECT [t0].[GroupID]
    FROM [ProcessRoundIssueInstance] AS [t0]
    GROUP BY [t0].[GroupID]
    ) AS [t1]

Find the answer here