Concatenate Unique Values After Lookup

I am trying to come up with a user defined function to accomplish several things at once.

First, I need to lookup a reference value in one column and determine the value from another column (on the same worksheet) in the same row. Then, concatenate each "return" value (that isn't blank).

I have the following code so far, but my formula results in a zero. If I remove the On Error Resume Next, the formula results in a #VALUE error.

Public Function ConcatUnique(Separator As String, Ref As Variant, LkupCol As Range, _
RetCol As Range)
Dim lkup As Range
Dim ret As Range
Dim colDif As Long
Dim mCollect As New Collection
Dim i As Integer
Dim b As Variant
' Determine the number of columns difference between
' the lookup column and the return value column.
colDif = RetCol.Column - LkupCol.Column
On Error Resume Next
' Determine which lookup values in the lookup range match the reference value.
' When the lookup value matches the reference value, set the return range object
' to the cell in the return range (column) in the same row as the lookup value.
' Note we use the difference between the lookup column and the return column to
' determine the location of the return range object.
For Each lkup In LkupCol
If lkup.Value = Ref.Value Then Set ret = Range(Cells(lkup.Row, lkup.Column _
+ colDif))
' Store the return value in the collection object. Ignore any blank return
' values. Note we use the range value converted to a string as the key
' value.
If ret.Value <> "" Then mCollect.Add ret.Value, CStr(ret.Value)
' Loop through each cell in the lookup column range.
Next lkup
' Write each item from the collection and the separator to the final result,
' writing each value and the separator after the previous value and separator.
For i = 1 To mCollect.Count
b = b & mCollect(i) & Separator
Next i
ConcatUnique = Left$(b, Len(b) - Len(Separator))
End Function