Document.HeaderText throws error

I am using Addon.Web and trying to implement HeaderText and FooterText to prevent users from modifying the namespaces in the root tag.

When I place the below in HeaderText, i get an error but no problem with the footer..myDoc.FooterText="</myTag>"; 

myDoc.HeaderText="<myTag xmlns:atom=\"http://www.w3.org/2005/Atom\" xmlns=\"http://www.mysite.ca/schemas/xsd/2.0\" xmlns:gp=\"http://www.mysite.ca/schemas/xsd/2.0/gp\">";

1.  I load the file
2. I strip the root tag to put in header
3. I strip end root tag for footer

Here is the error:

System.ArgumentOutOfRangeException was unhandled
  HResult=-2146233086
  Message=StartIndex cannot be less than zero.
Parameter name: startIndex
  Source=mscorlib
  ParamName=startIndex
  StackTrace:
       at System.String.InternalSubStringWithChecks(Int32 startIndex, Int32 length, Boolean fAlwaysCopy)
       at System.String.Substring(Int32 startIndex, Int32 length)
       at ActiproSoftware.SyntaxEditor.Addons.Xml.XmlSyntaxLanguage.ActiproSoftware.SyntaxEditor.ISemanticParserServiceProcessor.Process(SemanticParserServiceRequest request)
       at ActiproSoftware.SyntaxEditor.SemanticParserService.a(SemanticParserServiceRequest A_0)
       at ActiproSoftware.SyntaxEditor.SemanticParserService.c()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: