Usually that happens because the DLookup is trying to evaluate before the query or referenced control has valid data. A simple fix is to wrap the textbox control source with Nz() or an IIf(IsError(... )) check.

Something like this often works:
=Nz(DLookup("Fi eldName","YourQ uery","Criteria "),"")

That way the textbox shows blank instead of error speed stars until the query has...