I'm working in Excel 2002

I have a table with 18 columns where the user enters numeric values.  The
sheet is proctected, the cells the user inputs information into are unlocked.
Currently I have the following coding:

Private Sub Worksheet_Change(ByVal Target As Range)  
    If Target.Column = 18 Then
        Cells(Target.Row, 1).Select
    End If
End Sub

What this does is if the user tabs out of column 18, row 3 for example, the
cursor goes to column A in the same row.

The above coding works only if there is a numeric value in the cell in
column 18.  How can I get the above coding to work if the user leaves the
cell blank?

Thanks.