Parallel Transactions Error

Hi,All.
I'm writing test cases on C# for a few methods that make changes in database.To prevent making changes I used BeginTransaction-Rollback,everything was good.But this doesn't work if tested method has BeginTransaction-Rollback code itself.An error appears in NUnit: System.InvalidOperationException : SqlConnection does not support parallel transactions.
Do smb know how to solve the problem?

*******************************tc below********************
[Test]

[Category("Access")]

public void UpdateGroupUserTable(/*Int32 userID, GroupList dataset*/)

{

r.BeginTransaction();

try

{

r.UpdateGroupUserTable(userId,gl);

}

finally

{

r.Rollback();

}
*******************************tested method below********************

public void UpdateGroupUserTable(Int32 userID, GroupList dataset)

{

CheckDisposed();

UpdateCommand.CommandType = CommandType.Text;

UpdateCommand.Parameters.Clear();

UpdateCommand.Parameters.Add("@USERID", userID);

BeginTransaction();

try

{

Adapter.Update(dataset.acl_group);

Commit();

}

catch (SqlException)

{

Rollback();

throw;

}

}