type13 mismatch error message... any idea why?

Got this msg when I open the sheet... Gets into debugging of the following code:

Private Sub Worksheet_Calculate()

If Range("O16") <> "" Then
If Range("Q18") = "yes" Then
Range("O20") = Range("O16")
Range("O21") = Range("L16")
Range("P20") = Now
End If
End If
If Range("Q26") = "yes" Then
Range("O20") = ""
Range("O21") = ""
Range("P20") = ""
End If
End Sub

where is the mistake????? and it only monday...

Which line is causing the error?

Try this instead.

Private Sub Worksheet_Calculate()

If Range("O16").Value <> "" Then
     If Range("Q18").Value = "yes" Then
          Range("O20") = Range("O16")
          Range("O21") = Range("L16")
          Range("P20") = Now
     End If
End If

If Range("Q26").Value = "yes" Then
     Range("O20") = ""
     Range("O21") = ""
     Range("P20") = ""
End If

End Sub

thanks a lot Norie