Insert NULL into a Money field in SQL Server

I have a form for users to enter information. If they do not enter anything into txtPrice I want it to insert NULL into the column. The column is a money column. Everytime I try this is the error I get. Implicit conversion from data type nvarchar to money is not allowed. Use the CONVERT function to run this query. Here is the code I am using to insert the data entered into the text box.

If txtNPrice.Text = "" Then
                    cmdSelect.Parameters.AddWithValue("@necklacePrice", DBNull.Value)
                Else
                    cmdSelect.Parameters.AddWithValue("@necklacePrice", Val(txtNPrice.Text))
                End If

This is an ASP.NEt application written in vb.net.

Thanks