filtering (blank) from columnfield

[DANA]

I'm tring to remove the "(blank)" value from my columns values .

the field  "aircraft" has blank values and I want to ignore the blank column.

I've tried several things from code behind (working with vb.net) and nothing happens.

If I try filtering this field from the webpage, I get an error of :

"cannot convert object of type 'system.string' to type 'telerik.web.ui.pivotgrid.core.nullvalue'

<telerik:RadPivotGrid RenderMode="Lightweight" AllowPaging="true" PageSize="10" Height="500px"
              ID="RadPivotGrid1" runat="server" allowfiltering="true" emptyvalue="0">

              <Fields>

                  <telerik:PivotGridColumnField DataField="aircraft" showgroupwherenodata="false">
                  </telerik:PivotGridColumnField>
                  <telerik:PivotGridRowField DataField="Category" ZoneIndex="0">
                  </telerik:PivotGridRowField>
                  <telerik:PivotGridRowField DataField="ProductName" ZoneIndex="1">
                  </telerik:PivotGridRowField>

                  <telerik:PivotGridAggregateField DataField="emp_num" Aggregate="count" >
                  </telerik:PivotGridAggregateField>

              </Fields>
              <totalsettings rowgrandtotalposition="last"      />
          </telerik:RadPivotGrid>