Error in datetime field

Using VS2010, aspnet4, c #, SQLServer2008 and EF
I have a Person table, and a BirthDate field is of type date default null
When behest display table data in a grid eg date field and have not filled, gives the following error:
"Could not set property 'BirthDate' in 'Person' with a value 'null'.
This property must be set to a non-null value of type 'DateTime'. "
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