ACCESS 2000 - "Set rst" error message

On the last line  shown here I encounter an error message:
   Runtime error 91
   Object variable or With block variable not set
(I don't understand the message in this context.)

Public Sub CreateAndFormatDifFile()

    Dim db As DAO.Database
    Dim rst As DAO.Recordset
    Dim strSQL As String, strWriteFile As String, NewLine As String

    Set fs = CreateObject("Scripting.FileSystemObject")
   
    strWriteFile = "C:\Starbldr\AP_" & Format(Now(), "dd-mm-yy") & " " & Format(Now(), "h-mm-ss") & ".txt"
    Set ExpFile = fs.CreateTextFile(strWriteFile, True)

'Print the Header line
    DoCmd.OpenQuery "qryDeleteDIFHeader"
    DoCmd.OpenQuery "qryCreateDIFHeader"
   
    strSQL = "SELECT * FROM DIFHeader"
    Set rst = db.OpenRecordset(strSQL, dbOpenSnapshot)

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
Here is the entire procedure:

Public Sub CreateAndFormatDifFile()

    Dim db As DAO.Database
    Dim rst As DAO.Recordset
    Dim strSQL As String, strWriteFile As String, NewLine As String

    Set fs = CreateObject("Scripting.FileSystemObject")
   
    strWriteFile = "C:\Starbldr\AP_" & Format(Now(), "dd-mm-yy") & " " & Format(Now(), "h-mm-ss") & ".txt"
    Set ExpFile = fs.CreateTextFile(strWriteFile, True)

'Print the Header line
    DoCmd.OpenQuery "qryDeleteDIFHeader"
    DoCmd.OpenQuery "qryCreateDIFHeader"
   
    strSQL = "SELECT * FROM DIFHeader"
    Set rst = db.OpenRecordset(strSQL, dbOpenSnapshot)

    NewLine = Recty = rst![REC-TY]
    NewLine = NewLine & rst![CUST-VEND] & Space(6 - Len(rst![CUST-VEND]))
    NewLine = NewLine & rst![INVOICE] & Space(10 - Len(rst![INVOICE]))
    NewLine = NewLine & Space(15)
    NewLine = NewLine & rst![INV-DESC] & Space(24 - Len(rst![INV-DESC]))
    NewLine = NewLine & Space(4)
    NewLine = NewLine & rst![POST-CODE] & Space(4 - Len(rst![POST-CODE]))
    NewLine = NewLine & Space(8)
    NewLine = NewLine & rst![INV-DATE]
    NewLine = NewLine & rst![DUE-DATE]
   
    ExpFile.WriteLine NewLine
   
'Print the Detail lines
    DoCmd.OpenQuery "qryDeleteDIFDetail"
    DoCmd.OpenQuery "qryCreateDIFDetail"
   
    strSQL = "SELECT * FROM DIFDetail"
    Set rst = db.OpenRecordset(strSQL, dbOpenSnapshot)
   
    If rst.RecordsetCount > 0 Then
        rst.MoveFirst
        Do Until rst.EOF
            NewLine = Recty = rst![REC-TY]
            NewLine = NewLine & Space(5)
            NewLine = NewLine & rst![J-E-I-TYPE]
            NewLine = NewLine & rst![Job] & Space(10 - Len(rst![Job]))
            NewLine = NewLine & rst![PHASE] & Space(5 - Len(rst![PHASE]))
            NewLine = NewLine & rst![COST] & Space(5 - Len(rst![COST]))
            NewLine = NewLine & rst![COST-TYPE] & Space(2 - Len(rst![COST-TYPE]))
            NewLine = NewLine & Space(11)
            NewLine = NewLine & rst![ACCOUNT] & Space(8 - Len(rst![ACCOUNT]))
            NewLine = NewLine & Space(12 - Len(rst![DIST-AMT])) & rst![DIST-AMT]
            NewLine = NewLine & Space(42)
            NewLine = NewLine & rst![DIST-DESC] & Space(24 - Len(rst![DIST-DESC]))
                       
            ExpFile.WriteLine NewLine
            rst.MoveNext
        Loop
    End If
   
    rst.Close
    Set rst = Nothing
    db.Close
    Set db = Nothing

    ExpFile.Close
End Sub