Input string was not in a correct format. plz help

please help me , i get sick throgh building graduation project

and this problem "Input string was not in a correct format."

 

here part of coding

firstPage.. Search.aspx
Sub GetID(ByVal s As Object, ByVal e As GridViewCommandEventArgs) Handles GridView1.RowCommand
Dim RowIndex As Integer

RowIndex = 0
...