Error 13 type mismatch

[SlootsJ]

Private Sub Workbook_open()
   Dim r As Long
   Dim Msg As String
   With Sheets("HTL")
       For r = 11 To 69
           If .Range("C" & r).Value > 0 And .Range("AD" & r).Value < .Range("N" & r).Value Then
               Msg = Msg & vbLf & .Range("C" & r).Value
           End If
       Next r
       If Msg <> "" Then MsgBox Msg & vbLf & " Binnen uitstroom periode.", , "Let op!"
       End With
End Sub

When I run this code, I get a Error 13 Type Mismatch error.
It used to work, I did not change any values or types in the sheet.

 

[Joe4]

Do you any cells in columns C, N, or AD on your "HTL" sheet contain errors?