Getting "Could not cast or convert from System.String to System.DateTime" Exception if DataField is not entered from the user interface in Editable Grid

[n_s_adhikari@rediffmail.com]

I have a store defined as follows :

<ext:Store ID="storeRetailers" runat="server" OnBeforeStoreChanged="storeRetailers_BeforeChange">
        <Reader>
            <ext:JsonReader ReaderID="RetailerId">
                <Fields>
                    <ext:RecordField Name="RetailerId" />
                    <ext:RecordField Name="EngagedDate" Type="Date" />
                </Fields>
            </ext:JsonReader>
        </Reader>
    </ext:Store>

In the editable grid column config is as follows:

<ext:Column DataIndex="EngagedDate" Header="Engagement Date">
                                                <Renderer Format="Date" FormatArgs="'d/m/Y'" />
                                                <Editor>
                                                   <ext:DateField ID="txtEngagedDate" runat="server" AllowBlank="true" MsgTarget="Under" Format="dd/MM/yyyy"/>
                                                </Editor>
                                            </ext:Column>

But while saving the grid data, I am getting following exception If EngagementDate is not supplied from user interface.

Could not cast or convert from System.String to System.DateTime.

protected void contactStore_BeforeChange(object sender, BeforeStoreChangedEventArgs e)
    {
        ChangeRecords<ContactVO> data = e.DataHandler.ObjectData<ContactVO>(); // <-- Exception

    }

Can you please give me some clue to resolve this ?