Visual Studio 2008 Multiselection querry with LINQ Query

With Visual Studio 2008 and SQL Server 2005 I have a Form where i drag the  data Class (tbl_Name )  Linked to a Table  Named  tbl_Name

The Table has 5 Column : NameID/Name/Town/Country/Continent

So the Form show a Grid representing the tbl_Name with the 5 column
I have in same form Toolstrip    2 combo box named as follow :

ToolStripComboBox1    It Populate Taking Data from the column Continent
ToolStripComboBox2   It Populate Taking  Data from the collumn Country

 In the Form I have also a Button : Button3 that i use to trigger the filter action

I am using the following code to filter the Grid by Continent and by Country and it works very well

 Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click


        Dim MultyQuerry = From tbl_Name In db.tbl_Names _
                                Where tbl_Name.Continent = Me.ToolStripComboBox1.SelectedItem.ToString _
                                And tbl_Name.Country = Me.ToolStripComboBox2.SelectedItem.ToString _
                                Select tbl_Name
        Me.Tbl_NameBindingSource.DataSource = MultyQuerry
End Sub

Now let me come to my Problem

All is working if i have both Combo Box with a value :

But it give me error if one of the comboBox is empty

Is anyone helping me to change my code so to handle the Null value ???

Thanks