Unable to cast object error when looping thru table

What is wrong with either of these?

I receive the following when i debug:

Unable to cast object of type 'System.Web.UI.LiteralControl' to type 'System.Web.UI.WebControls.TextBox

TextBox txtQty = (TextBox)row.Cells[1].Controls[0];
                TextBox txtPart = (TextBox)row.Cells[2].Controls[0];
                TextBox txtUnit = (TextBox)row.Cells[3].Controls[0];


                var txtQty = (TextBox)row.Cells[1].Controls[0];
                var txtPart = (TextBox)row.Cells[2].Controls[0];
                var txtUnit = (TextBox)row.Cells[3].Controls[0];