Can't get the information from my membership database

 Hi,
for some reasone I can't get the information from my membership things. I need to get the UserID for the person that have logged in and trying to buy something so I can put it in the next database. However, from the code:
param = comm.CreateParameter();
param.ParameterName = "@CustomerID";
param.Value = Membership.GetUser(HttpContext.Current.User.Identity.Name).ProviderUserKey.ToString();
param.DbType = DbType.Guid;
param.Size = 16;
I get the errormessage "System.InvalidCastException: Specified cast is not valid."
I have no ide if this is the right forumpart, didn't find anyone that semed to match. Can someone please help me, as fast as possible, I've got customers waiting :) 
// Namirna

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