Unhandled exception when code runs for the first time,but works fine the second

Question

Hi All,
I have the following code snippet
private void RGL_Click(object sender, RibbonControlEventArgs e)
Word._Document oDoc;
           oDoc = Globals.ThisAddIn.Application.ActiveDocument;
            object start = oDoc.Content.Start;
            object end = oDoc.Content.End;
            oDoc.Range(ref start, ref end).LanguageID = WdLanguageID.wdEnglishCanadian;
            oDoc.Range(ref start, ref end).NoProofing = 0;
Microsoft.Office.Interop.Word.ReadabilityStatistic DocStats;
                DocStats = oDoc.ReadabilityStatistics[10];------>exception occurs here when code runs first time
                MessageBox.Show(DocStats.Name + " " + DocStats.Value);
When I copy paste a document and click on the button it raises a COM exception but if I click continue and click on the button again, the correct grade level score is displayed.Cant figure out why this is happening.

Answer 1

Thanks for the answer Eugene, this exception never shows up if I type something and click on the rgl button , but if i open a huge document that is already in Canadian english and click on this ribbon it gives me the following exception
System.Runtime.InteropServices.COMException was unhandled by user code
  HelpLink=wdmain11.chm#36966
  HResult=-2146824090
  Message=Command failed
  Source=Microsoft Word
  ErrorCode=-2146824090
  StackTrace:
       at Microsoft.Office.Interop.Word.ReadabilityStatistics.get_Item(Object& Index)
       at UsabilityMapping_V01.Ribbon1.RGL_Click(Object sender, RibbonControlEventArgs e) in <my path> 1211
       at Microsoft.Office.Tools.Ribbon.RibbonPropertyStorage.ControlActionRaise(IRibbonControl control)
       at Microsoft.Office.Tools.Ribbon.RibbonPropertyStorage.ButtonClickCallback(RibbonComponentImpl component, Object[] args)
       at Microsoft.Office.Tools.Ribbon.RibbonManagerImpl.Invoke(RibbonComponentCallback callback, Object[] args)
       at Microsoft.Office.Tools.Ribbon.RibbonMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
       at Microsoft.Office.Tools.Ribbon.RibbonManagerImpl.System.Reflection.IReflect.InvokeMember(String name, BindingFlags invokeAttr, Binder binder, Object target, Object[] args, ParameterModifier[] modifiers, CultureInfo culture, String[]
namedParameters)
  InnerException: