System.FormatException: Input string was not in a correct format

Hello

I have a problem with this script. Whats is wrong with it?

Function OK()


Workbooks.Open Filename:="C:\Rapport_2012-04-04.csv"


Sheets("Rapport").Activate
    Dim what3 As String = "OK"
      Do
      Set r1 = ActiveSheet.UsedRange.Find(what3)
        If r1 Is Nothing Then
            Exit Do
          Else
            Rows(r1.Row).Delete
        End If
    Loop


End Function

The error message is

{returnCode=200;sessionId=61388600-3ed8-42f6-b502-d59efe08aabd;exception=System.FormatException: Input string was not in a correct format.
at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
at dotNETWebActions_RunScript.RunScriptFromFile.Execute(ActionRequest req, ISession s, IActionRegistry reg)
at com.iconclude.agent.ActionProxy.Execute(ActionRequest cmd, ISession s, IActionRegistry reg);}

Why not in correct format ? everything looks fine.

Best regards
L