DocLinks created in 6 don't work in 5

A link created in an R6 application with AppendDocLink does not work in an R5 client. If I open the same mail database in R6 the link works fine. Are there any co-existing issues with AppendDocLink. Code below for reference:

TIA

Set RTItem = New NotesRichTextItem(curdoc, "DocLink")

Call RTItem.AppendDocLink(curdoc,"")

Set ddate = curdoc.ReplaceItemValue( "DiscSubmitted", Now() )

Call uidoc.save

Call curdoc.send (True)

Call uidoc.save

Call uidoc.close