"application-defined Or User-defined Error" When Applying Validation In Vba

I've made a macro that inserts an entire new row above a specified cell, then updates each column of that row using offset.value and various constants and inputbox variables. Works great so far, unless I try to apply a validation rule to one of the cells.

The code I'm using to add the validation works okay when referring to a specific cell by reference e.g:


With Range("A1").Validation

etc...

However, when I change it thusly:


With Sheet2.Range("LastRow").Offset(-1, 5).Validation
.Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
xlBetween, Formula1:="=Departments"
.IgnoreBlank = True
.InCellDropdown = True
End With

It doesn't like that one bit.

This is part of a private Sub attached to a button in the same sheet. I've had a scan through the archives looking for validation threads, but couldn't find anything that refers to problems with offsets (if indeed that is the problem here).