Datastore entity with kind and key has a null property named

After adding a boolean attribute to an obj开发者_JAVA百科ect, fetching that object from the datastore results in the error:

Datastore entity with kind <Class> and key <Class:ID> has a 
  null property named <attribute>

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Don't use primitive data-types, use their wrapper classes.
Instead of boolean, use Boolean, etc.