Runtime Error 91: Object variable or With block variable not set

Hi all,
I have a macro that performs filtering and deleting unwanted rows.
I find it strange that it is now not working when it was working few days ago.
It's giving me a runtime error 91.

Here's my code:

Sub Testing()
    
    Dim Cl     As Range
    Dim rng    As Range
    Dim R      As Long
    Dim X      As Long

    R = ActiveSheet.Cells(Rows.Count, "K").End(xlUp).Row
    For X = 2 To R
        If Cells(X, "K").Value <> "AB," And Cells(X, "K").Value <> "CD," And _
         Cells(X, "K").Value <> "EF," And Cells(X, "K").Value <> "IJ," And Cells(X, "K").Value <> "TU," Then
            If rng Is Nothing Then
                Set rng = Cells(X, "K")
            Else: Set rng = Union(rng, Cells(X, "K"))
            End If
        End If
    Next X
    
    rng.EntireRow.Delete


End Sub

Any ideas? Appreciate any help. Thank you.