Excel vba Function with return

I found this solution for my problem. And it works fine like this:

Dim CNPJs As New Collection, a
Dim i As Long
Dim rng As Range

U_L = Plan2.Range("A" & Rows.Count).End(xlUp).Row

On Error Resume Next
For Each rng In Plan2.Range("A2:A" & U_L)
    CNPJs.Add rng.value, rng.value
Next

But now I want to bulid a function that returns this unique array, because I will use this many times... I tried this:

Public Function unArray(myRange As Range) As Collection

    Dim arr As Collection, a

    On Error Resume Next
    For Each rng In myRange
        arr.Add rng.value, rng.value
    Next

    unArray = arr

End Function

And I call it:

Sub test()

      Dim CNPJs As New Collection, a

      U_L = Plan2.Range("A" & Rows.Count).End(xlUp).Row

      CNPJs = unArray(Plan2.Range("A2:A" & U_L))

End Sub

But it returns this error:

"Compilation error:"
"The argument is required"

Can you help me?