I got this COM exception when running this code(in C#)

object startPos = (object)start;
object endPos = (object)end;
Word._Document doc = .... // all these have been initialized
doc.Range( ref start, ref end );

the last line gives me "Value out of range" exception if the endPos is
greater than doc.Content.End

How should i expand the document content to accommodate that?
Thanks in advance!