InvalidCastException when getting first result from LINQ query

[RobR]

Greetings!

I am a complete newbie at LINQ. I am trying to get my first simple little application using it to run. It's built in Visual Studio 2008, using DevArt's LinqConnect product to talk to a PostgreSQL database. Here's the function:

		private void dgvCharges_CellClick(object sender, DataGridViewCellEventArgs e)
		{
			long chargeNumber = Convert.ToInt32(dgvCharges[e.ColumnIndex, e.RowIndex].Value);
			var oneChargeSet = from charge in m_context.Charges
							where charge.Charge1 == chargeNumber
							select charge;
			int chargeCount = oneChargeSet.Count();
			var oneCharge = oneChargeSet.First();                      
		}

In the debugger, I see that chargeCount is 1, as expected. I believe that the First() function should return an object that contains all of the data members of the charge table in the database: Charge1, BuiltDate, LoadedDate, Cycle, and everything else. This code compiles and starts nicely. But when I invoke the First() method, I get an InvalidCastException. I get the same expression when I try to give First an explicit type:

		private void dgvCharges_CellClick(object sender, DataGridViewCellEventArgs e)
		{
			long chargeNumber = Convert.ToInt32(dgvCharges[e.ColumnIndex, e.RowIndex].Value);
			var oneChargeSet = from charge in m_context.Charges
							where charge.Charge1 == chargeNumber
							select charge;
			int chargeCount = oneChargeSet.Count();
			var oneCharge = oneChargeSet.First<AnnealContext.Charge>();
		}

Obviously, I don't understand something basic about LINQ. How do I get this to work?

Thanks very much!

RobR