Why I am not able to assign Default value ot my property using MVC2 in my asp.net mvc application

I have this code in my class

[MetadataType(typeof(HipaaBenefit_Metadata))]
    public partial class HipaaBenefit
    {
        public class HipaaBenefit_Metadata
        {
            [DefaultValue("")]
            public string TimePeriodQualifierEb06 { get; set; }

            [DefaultValue("")]
            public string NetworkIndicatorEb12 { get; set; }

            [DefaultValue("")]
            public string BenefitAmountEb07 { get; set; }
        }
    }

In my Database table these columns are not null fields. when I am trying to save the data from codebehid to table I a开发者_StackOverflowm trying to assign these column values as Empty.String.

But some reasons I am not able to send this Empty.String values to the table?

is that something I am doing wrong in this code? if so how to assign the default values?

Thanks

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Did you tryied to set default value as NULL?

[DefaultValue("Null")]