Error "value does not fall within expected range" while accessing the field value of a list column in SharePoint 2010

Hi,

I have a list as PODetails which have a field called PONumber of(data type) Single Line of Text. I have created an application page where I am accessing field value in a button click event. Everything is fine. The whole code is running well but when I am trying to retrieve the field value it showing the error "Value does not fall within expected range". The code is "String strPONum =item["PONumber"].ToString();".

I went through a number of blogs and forums and checked all the possible solution but nothing worked out.

Please help me out. I am using SharePoint 2010 and Visual Studio 2010.

Thanks In Advance

Sidhanta