Error:unable to cast object of type

[C# Programming]

Hi.
i face a Error
Unable to cast object of type 'System.Windows.Forms.TextBox' to type 'System.IConvertible'.

When I Do this

int ProdTotal=0;
ConTotalTxt.Text = Convert.ToString(Convert.ToInt32(ProdTotTxt) + ProdTotal);

in this code where i do mistak???

thanks in Advance

Jawad Khatri