Try getting a Value and also compare it first with DbNull.Value

>I'm trying to add a Beneficiary to an Insurance Policy using an mmGridView. I've done this type of thing before without this error, so I'm pretty sure it's something silly I'm missing.
>
>The line of code that is crashing is this:
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Guid newPK = (Guid)dsBeneficiary.Tables[0].Rows[dsBeneficiary.Tables[0].Rows.Count - 1]["BeneficiaryPK"];

>
>just prior to this line I do a:
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this.oBeneficiary.NewRow(dsBeneficiary, this.oBeneficiary.TableName);

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>My Beneficiary .cs sets a default BeneficiaryPK using:
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entity.BeneficiaryPK = Guid.NewGuid();

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>The Watch window shows {} as the value for dsBeneficiary.Tables[0].Rows[dsBeneficiary.Tables[0].Rows.Count - 1]["BeneficiaryPK"].
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>The full error is this:
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System.InvalidCastException was unhandled by user code
>  Message="Specified cast is not valid."
>  Source="SamaanSystems.IAS.Web"
>  StackTrace:
>       at SamaanSystems.IAS.Web.PolicyEdit.btnAddBeneficiary_Click(Object sender, EventArgs e) in C:\Development\Mere Mortals .NET Framework 2008\SamaanSystems\IAS.Web\PolicyEdit.aspx.cs:line 592
>       at System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e)
>       at System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument)
>       at System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
>       at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
>       at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
>       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
>  InnerException:

>
>Any ideas what I have done wrong?