GUID Vs SelectFieldValue

Hello Customer Service,
I have a relationship table indexed on UserID (type=GUID) and ClinicID (type=Int32. I want to select the single row as follows:
Guid userId = ((Guid)(SelectFieldValue("UserId")));
Int32 clinicId = (Convert.ToInt32(SelectFieldValue("ClinicId")));
ClinicRight cr = ClinicRight.SelectSingle(userId, clinicId);

The line, Guid userId = ((Guid)(SelectFieldValue("UserId"))); gives an invalid GUID cast error. What’s the best way to cast this user Ids?
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