Need help quick .......

I have a flexgrid where the first column holds numbers.

The code below shows tooltiptext based on these numbers when you move the mouse over them.

With the following code, how can I stop getting integer overflow errors when the mouse moves over the flexgrid where there is no valid line ? What I mean is the maximum number of rows which can be displayed at once is 15, but if only the first 2 or 3 are populated, this leaves a 'grey area' and the code below can't handle that, and if the mouse moves in this 'grey area' I get integer overflow errors at the line marked with an arrow.

Can anyone help me out ? I need to sort this quick, so any input would be very much appreciated.

Thanks in advance.

VB Code:

Private Sub flxDetails_MouseMove(Button As Integer, Shift As Integer, x As Single, y As Single)
 
    Static SelectedRow As Long
    Dim rowNum As Long
    Dim colNum As Long
    Dim z As Integer
    Dim BrnName As String * 30
    
        
    If ReadyToProceed Then
        With flxDetails
            rowNum = y \ .RowHeight(1) + .TopRow + 1
            colNum = x \ .ColWidth(1) + .LeftCol + 1
        
        
                If rowNum <> SelectedRow And colNum = 1 Then
                    SelectedRow = rowNum
                    For z = 0 To 999
      -->                 If .TextMatrix(rowNum - 2, 0) = tmpBRN(z) Then
                            BrnName = tmpBrnName(z)
                            x = 999
                        End If
                    Next z
                    
                    .ToolTipText = Trim(BrnName)
                    
                    If .TextMatrix(rowNum - 2, 0) = "050" Then
                        .ToolTipText = "Nantgarw Warehouse"
                        Exit Sub
                    End If
                
                End If
        End With
    End If
 
End Sub