Fill In A Cell Before Save

I am creating a form for others to fill in. But they don't fill it completely.
So i want to force them to

If they fill in Cell G31 or H31 they must also fill in cel F31, before it will save the document.

If they try to save the file without F31 filled, it should give a messagebox that
says: Fill in Volume.

Privatesub Workbook_BeforeSave(ByVal SaveAsUi As Boolean, Cancel As Boolean)

Dim msg As String

If Not Worksheets("Sheet1"). Range("G31").Value Like "##" then
msg = "Befor saving, fill in Volume."
Cancel = True
End If

If Not Worksheets("Sheet1").Range("H31").Value Like "##" Then
msg = msg & vbCrLf & "Before saving, fill in Volume."
Cancel = True
End If

If Cancel Then
MsgBox msg
End If
End Sub