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

Question

I am trying to update a template in a gridview but it gaves me this error:

 

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

 

Below is my code behind.

int rowIndex = ((GridViewRow)((LinkButton)e.CommandSource).NamingContainer).RowIndex;

int id = Convert.ToInt32(e.CommandArgument);
string option = ((TextBox)GridView1.Rows[rowIndex].FindControl("TextBox1")).Text;
string day = ((TextBox)GridView1.Rows[rowIndex].FindControl("TextBox2")).Text;
decimal netpremium = Convert.ToDecimal(((TextBox)GridView1.Rows[rowIndex].FindControl("TextBox3")));
decimal vat = Convert.ToDecimal(((TextBox)GridView1.Rows[rowIndex].FindControl("TextBox4")));
decimal registry = Convert.ToDecimal(((TextBox)GridView1.Rows[rowIndex].FindControl("TextBox5")));
decimal totalpremium = Convert.ToDecimal(((TextBox)GridView1.Rows[rowIndex].FindControl("TextBox6")));
TravelAssuranceDataAccess.UpdatePremium(id, option, day, netpremium, vat, registry, totalpremium);
GridView1.EditIndex = -1;
BindGridViewData();

Thanks in advance for any assistance.