Error while trying to add new objects

Jul 11, 2012 at 9:28 AM

I get following error while trying to insert second object(a Class that has list of other classes inside it as list):

Error code:

"An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll"

My development environment: Windows Phone 7.1, Microsoft Visual studio 2010.

Is it known error? If not I can add more details.

Many thanks in advance.