ArrayList Sort, InvalidOperationException

Hi, I sometimes get InvalidOperationException when sorting an arraylist object. Is there any reason why its happens sometimes. following is the stack trace of the error.

System.InvalidOperationException was unhandled
Message="InvalidOperationException"
StackTrace:
at SorterObjectArray.QuickSort()
at System.Array.Sort()
at System.Array.Sort()
at System.Collections.ArrayList.Sort()
at System.Collections.ArrayList.Sort()
at localize.Form1.computeposition()
at localize.Form1.proceed_Click()
at System.Windows.Forms.Control.OnClick()
at System.Windows.Forms.Button.OnClick()
at System.Windows.Forms.ButtonBase.WnProc()
at System.Windows.Forms.Control._InternalWnProc()
at Microsoft.AGL.Forms.EVL.EnterMainLoop()
at System.Windows.Forms.Application.Run()
at localize.Program.Main()

any suggestions would be helpful.

Thanks you!!!