Gallery Lookup across 2 SharePoint Lists - Lookup error on blank rows

[KeithD]

I have a gallery in place that was originally just looking at 1 SharePoint list how ever i want to expand what it is showing slightly with some data coming from a 2nd SharePoint list so have changed the Items property of the gallery to something like this

AddColumns(List1, "IssuedUser", Lookup(List2, 'field_1' = IssuedClockNo).'field_2' & " " & Lookup(List2, 'field_1' = IssuedClockNo).'field_3'

All my old columns are populated OK, the new column "IssuedUser" populates fully for all rows with an IssuedClockNo,

If a stamp is not in use IssuedClockNo is empty

I am seeing 'The requested operation is invalid, server response list2 failed fetching items failed, possible invalid string in filter query ....'

I assume i need to carry out a check if IssuedClockNo is empty before doing the lookup ?

What would be the best way of doing this?