Code will only execute from one sheet

Hello, I have read this user form code eight thousand times and cannot find what I am missing but this code currently will only run if the active sheet is "DATABASE". If I attempt to run it with another sheet active I get Run Time Error 1004: Application-defined or object-defined error.

Const db As String = "Customer Equipment Database.xlsm"

Private Sub OKButt_Click()

If MultiPage1.Value = 0 Then MsgBox CSelectBox.Value

If MultiPage1.Value = 1 Then    'ID number tab is selected
  Dim RowRange As Range
  Dim RowCnt As Integer, r As Integer
  RowCnt = 0
  For r = 0 To PSelectBox.ListCount - 1
    If PSelectBox.Selected(r) Then
        RowCnt = RowCnt + 1
        If RowCnt = 1 Then
            Set RowRange = Workbooks(db).Worksheets("DATABASE"). _
              Range("A2", Range("A2").End(xlDown)).Rows(r + 1)
        Else
            Set RowRange = _
              Union(RowRange, Workbooks(db).Worksheets("DATABASE"). _
              Range("A2", Range("A2").End(xlDown)).Rows(r + 1))
        End If
    End If
  Next r
If Not RowRange Is Nothing Then RowRange.Select
End If
Unload Me

End Sub

Private Sub UserForm_Initialize()

'POPULATE CUSTOMER SELECTION BOX
Dim cell As Range
Dim NoDupes As New Collection
Dim Custs As Range: Set Custs = Workbooks(db).Worksheets("DATABASE"). _
                         Range("A2", Range("A2").End(xlDown))
Dim vaItems As Variant

'add unique customer names to new collection
On Error Resume Next
For Each cell In Custs
    NoDupes.Add cell.Value, CStr(cell.Value)
Next cell
On Error GoTo 0

'add the customers to the box
CSelectBox.RowSource = ""
For Each Item In NoDupes
    CSelectBox.AddItem Item
Next Item

'sort and re-add customers to the box
vaItems = CSelectBox.list
For i = LBound(vaItems, 1) To UBound(vaItems, 1) - 1
    For j = i + 1 To UBound(vaItems, 1)
        If vaItems(i, 0) > vaItems(j, 0) Then
            vTemp = vaItems(i, 0)
            vaItems(i, 0) = vaItems(j, 0)
            vaItems(j, 0) = vTemp
        End If
    Next j
Next i
CSelectBox.Clear
For i = LBound(vaItems, 1) To UBound(vaItems, 1)
    CSelectBox.AddItem vaItems(i, 0)
Next i
'*****************************************************************************************

'POPULATE ID SELECTION BOX
Dim rng As Range
Set rng = Workbooks(db).Worksheets("DATABASE").Range("A2:C2", Range("A2:C2").End(xlDown))
With PSelectBox
    .ColumnCount = 3
    .RowSource = rng.Address
    .ListIndex = 0
End With

End Sub