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:

Guid newPK = (Guid)dsBeneficiary.Tables[0].Rows[dsBeneficiary.Tables[0].Rows.Count - 1]["BeneficiaryPK"];

just prior to this line I do a:

this.oBeneficiary.NewRow(dsBeneficiary, this.oBeneficiary.TableName);

My Beneficiary .cs sets a default BeneficiaryPK using:

entity.BeneficiaryPK = Guid.NewGuid();

The Watch window shows {} as the value for dsBeneficiary.Tables[0].Rows[dsBeneficiary.Tables[0].Rows.Count - 1]["BeneficiaryPK"].

The full error is this:

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?