msgbox: Argument not optional

Hi,

I want a msgbox to appear if there are blank cells in a range,
the code below is nearly there, however with teh msgbox I receive:
'Compile Error: Argument not optional'.
Many thanks in advance

Sub budgettemplate()


lastrow = ActiveSheet.UsedRange.Rows.Count
MsgBox lastrow

For i = lastrow To 1 Step -1
For j = 19 To 1 Step -1
If Cells(i, j) = "" Then
Cells(i, j).Interior.ColorIndex = 3
MsgBox 'You have blank cells, see shaded area'

End If
Next
Next

End Sub