LINQ error: Null value for non-nullable member.

When i tried to save an object with a complex type (with a null instance of the complex type), i got the following error:

{"Null value for non-nullable member. Member: 'Sentence'."}

In this link there is an explanation for this: http://social.msdn.microsoft.com/Forums/en-US/adonetefx/thread/50d33271-c8e3-468f-82e1-7c3178cb4322/
According to the explanation, complex types are always considered as required in entity framework. My workaround is creating an instance of the complex type with new keyword in the constructor.