Forms Data Controls :: Error In Editing DetailsView With Dropdownlist

ASP.NET - C# (C Sharp) - SQL Server

I've got a master table and a child table. The master table has a nullable column called ReferenceID and in some cases it should be NULL, but when it's null in a record, I get an error while going to Edit mode of DetailsView.

DB Tables:
MasterTable [Columns: MasterTableID (PK), Name, ReferenceID (FK)(Nullable)]
ReferenceTable [Columns: ReferenceID (PK), Reference]
<asp:DetailsView ID="DetailsView1" runat="server" AutoGenerateRows="False">
<Fields>
<asp:BoundField DataField="MasterTableID" HeaderText="MasterTableID"/>
<asp:BoundField DataField="Name" HeaderText="Name"/>
<asp:TemplateField HeaderText="Reference" SortExpression="Reference">
<EditItemTemplate>
<asp:DropDownList ID="DropDownList_Reference" runat="server"
DataSourceID="ObjectDataSource_ReferenceTable" DataTextField="Reference"
DataValueField="ReferenceID" SelectedValue='<%# Bind("ReferenceID") %>'>
</asp:DropDownList>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="LabelReference" runat="server"
Text='<%# Bind("Reference") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:CommandField ShowDeleteButton="True" ShowEditButton="True" />
</Fields>
</asp:DetailsView>
Select Queries:
Select MasterTable.*, ReferenceTable.Reference
From MasterTable INNER JOIN
ReferenceTable ON MasterTable.ReferenceID = ReferenceTable.ReferenceID
Select * from ReferenceTable

In Read Only mode all everything is OK, but when I click on Edit and Postback happens, I get the following error: Server Error in '/Sample' Application. 'DropDownList_Reference ' has a SelectedValue which is invalid because it does not exist in the list of items.Parameter name: value It means that in MasterTable, the ReferenceID shouldn't be NULL, but it should be in some cases.