MongoDb C # driver 2.0 Using BsonNull

I have a property isDeleted

Nullable

in a class Profile

.

Builders<Profile>.Filter.Eq(p => p.IsDeleted, BsonNull.Value)

      

        
        
        
      

    

But the following code caused the following compilation error:

Error 11    Cannot convert lambda expression to type 
'MongoDB.Driver.FieldDefinition<MongoDB.DataTypes.Profile,MongoDB.Bson.BsonNull>' 
because it is not a delegate type   

      

        
        
        
      

    

How do I implement null validation?