How can I remove effects of text properties in 'before-string

[Amos Bird]

Hi there,

emacs -Q

(set-window-margins (get-buffer-window) 20 0)
(overlay-put (make-overlay (point) (point)) 'before-string 
(propertize "." 'display `((margin left-margin) ,"Hi there!")))
(hl-line-mode)

The string "Hi there!" is also highlighted. How can I disable 
that?

regards,
--
Amos Bird
amosbird@gmail.com