With something like state I would just leave it as a short text and store the actual value. There aren't very many and you aren't going to be adding more or changing them (unless you go worldwide and store them in that combo). One less relationship to deal with, but it's fine if you do that.

There is no CustomerFrm in the downloadable database so could you share the whole code section related to that sub you have so we know which part is erroring?

Adam is correct (like usual). Without looking at the code, I guess "S" is defined as a String and now you are assigning a Number to it.

It didn't have to be States.  It could have been Desserts, Starships, Car Models, Candy, or something where you likely wouldn't use a String but rather a number.  This video is main reason I finally caved in and joined.  Other videos had almost tipped me into joining.  And I have learned a lot so far, and yes I'm going to rewatch the 4 hour beginner course (which I watched earlier on YouTube).  But I'm back again where I started because I love banging my head on a brick wall :).

My original coding was closer to what somebody at Stackoverflow.com did, and it snaps/dies/crashed at the same spot.  I like Richards approach as the code looks cleaner and .

I can now see why some coders don't like Multi-Select List Boxes with a passion, and saw some code that worked with numeric values but a.) I can't find that code sample at this time and b.) when I saw the code previously I couldn't wrap my head around it.

What is stored in S?

In Richards code S is a String

It's the S = "SELECT * FROM CustomerT Where StateID IN (" & S & ")"  

I Think I found another way, It's using a Form's filter rather than a Query.... but I really really really wanted to use a Query instead.

I found a way to do it.

On my form with the list box the VBA is

Private Sub CustomerListBtn_Click()
    On Error GoTo Err_cmdFilter_Click
    Dim i As Variant
    Dim StrIN As Variant
    
    StrIN = ""
    For Each i In StateList.ItemsSelected
        StrIN = StrIN & StateList.ItemData(i) & ","
    Next i
    StrIN = Left(StrIN, Len(StrIN) - 1) ' remove trailing comma.
   ' Set the Filter Text that will be called with the CustomerListQ2Frm Opens
    Me.MyFilterTxtBx = "StateID IN(" & StrIN & ")"
  
    DoCmd.OpenForm "CustomerListQ2Frm"

Exit_cmdFilter_Click:
    Exit Sub

Err_cmdFilter_Click:
Select Case Err.Number
    Case 5
        MsgBox "Please select 1 or more items from the listbox", vbOKOnly
    Case Else
        MsgBox Err.Number & "-" & Err.Description
End Select
Resume Exit_cmdFilter_Click

End Sub

The other form opens the Form Load event checks to see if that form is open and if it is, it borrows the location and sets the filter condition and turns the filter.  If it's not open it doesn't matter.
  
Private Sub Form_Load()
    Dim StrIN As Variant
    'Check to See if ListBoxFrm is loaded
    If CurrentProject.AllForms("ListBoxFrm").IsLoaded Then
        MsgBox ("The Form ListBoxFrm is Loaded")
        StrIN = Forms!ListBoxFrm!MyFilterTxtBx
        Me.Filter = StrIN
        Me.FilterOn = True
    End If

End Sub