Error converting from String to Int in TextBox

I have a GridView and inside this I have the function RowUpdating and when I want to change the data, I throw a conversion error.

  

Input string was not in a correct format.

In the way I want to convert the TextBox is the following:

int id = Convert.ToInt32(((TextBox)row.FindControl("txtid")).Text);

A Tip that gives me is the following:

  

When converting to string to DateTime, parse the string to taje the date berfore putting each Variable into the DateTime object.

but obviously I do not want to convert it to DateTime since it is a Id .