InvalidOperationException-in-Quotes-window

CODE:
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at WealthLab8.cwQuotes.<>c__DisplayClass62_0.CompareStub()
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)