Can someone tell me whats wrong with this asp.net code?

<asp:TemplateColumn HeaderText="ack" Visible="True">
<ItemTemplate>
<%# if DataSet1.FieldValue("CALLACK", Container) = true then %>
<img align="middle" src='image1.gif'>
<%else%>
<img align="middle" src='iamge2.gif'>
<%end if%>
</ItemTemplate>
</asp:TemplateColumn>

I keep getting the error message :

"Expression expected"