Image in datagrid

I have a field in a dataset called CALLACK which contains either 1 or 0.

In my datagrid I wish to display an image based upon this value. To do this I use itemtemplate :

<asp:TemplateColumn HeaderText="" HeaderStyle-Width="25" Visible="True">
<ItemTemplate>
<asp:image runat="server" ImageUrl='<%# SetDisImage(Databind.Eval Container,"DataItem.dbo.view_customers_calls.CALLACK"))%>'>
</asp:image>
</itemtemplate>
</asp:TemplateColumn>

The SetDisImage function is as follows:

Protected Function SetDisImage(ByVal bol As Boolean)
Dim strURL = ""
Dim strYes = "status_new.gif"
Dim strNo = "status_open.gif"

If bol Then
strURL += strYes
Else
strURL += strNo
End If
Return strYes
End Function

WHen I run the page I get the following error:

Expression does not produce a value.

Can someone please tell me what I am doing wrong. The code I got from the web and it seems pretty clear to me.

Thanks in Advance

VB Code:

Protected Function SetDisImage(ByVal bol As Boolean) 
        Dim strURL = "" 
        Dim strYes = "status_new.gif" 
        Dim strNo = "status_open.gif" 
 
        If bol Then 
             strURL += strYes 
        Else 
             strURL += strNo 
        End If 
        Return [b]strURL[/b] ' Could it be this?
End Function

I changed it just for testing purposes.

If I use :

<asp:image runat="server" ImageUrl='<%# SetDisImage(1)%>'>

it works fine. There must be something wrong with the parameter but I dont know what.