diff --git a/docs/users_guide/external_core.xml b/docs/users_guide/external_core.xml
index bc9d350d34fdbb67abda19226540d7cd22e29ae3..22273818da7d286be8d08bed43ee50269e856033 100644
--- a/docs/users_guide/external_core.xml
+++ b/docs/users_guide/external_core.xml
@@ -1786,7 +1786,7 @@ case (error "") of
         <state>South Carolina</state>
       </address>
       <pubdate>1993</pubdate>  
-      <bibliomisc><ulink url="citeseer.ist.psu.edu/launchbury93natural.html">citeseer.ist.psu.edu/launchbury93natural.html</ulink></bibliomisc>
+      <bibliomisc><ulink url="http://citeseer.ist.psu.edu/launchbury93natural.html">http://citeseer.ist.psu.edu/launchbury93natural.html</ulink></bibliomisc>
       <!-- booktitle = "Conference Record of the Twentieth Annual {ACM} {SIGPLAN}-{SIGACT} Symposium on Principles of Programming Languages", -->
     </biblioentry>