Encountering A Problem When Adding A Textbox Value To A DB Value

 hi,
I'm using the following code to add a textbox and a column in a DB.......................
 
SqlConnection objConn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
SqlCommand objCmd = new SqlCommand("UPDATE Cash SET Cash_Balance=Cash_Balance+@myValue", objConn);
objCmd.Parameters.AddWithValue("@myValue", txtAMST.Text);
objConn.Open();
objCmd.ExecuteNonQuery();
objConn.Close();
I have it almost working, except for 1 issue. The value in the DB column is 400. The value in the textbox is, for example, 200. When the code executes, instead of putting in 600 in the DB column, it puts in 400200. Any idea as to what i'm leaving out?