Conditional Concatenate Function: References ConcatenateIf Displays "#NAME?"

I have two tabs that reference the function ConcatenateIf(), defined by me, see below:

Function ConcatenateIf(iRange As Range, iLook As String, iNum As Integer)
For Each cell In iRange
If cell.Value <> iLook Then
ConcatenateIf = ConcatenateIf & Chr$(10) & cell.Offset(0, iNum).Value
End If
Next cell
End Function

Works great. But then, I copy one of the tabs to make a third tab, and suddenly every cell in all three spreadsheets that references ConcatenateIf displays "#NAME?".