System.FormatException: Input string was not in a correct format.

I get the following error when I run my application. It was working fine in 3.5. I am facing this issue when we migrated to .Net 4.0. Has anything changed in 4.0 types or something else which is causing this?
Any response is greatly appreciated in advance.
[FormatException: Input string was not in a correct format.] System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) +9594283 System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) +119 System.Convert.ToInt32(String value, IFormatProvider provider) +48 System.Web.UI.WebControls.GridView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup) +1047 System.Web.UI.WebControls.GridView.RaisePostBackEvent(String eventArgument) +210 System.Web.UI.WebControls.GridView.System.Web.UI.IPostBackEventHandler.RaisePost ...