--- a/src/Doc/Locales/document/root.bib	Wed Jul 18 12:21:55 2018 +0200
+++ b/src/Doc/Locales/document/root.bib	Wed Jul 18 16:44:01 2018 +0200
@@ -1,7 +1,7 @@
 @unpublished{IsarRef,
   author = "Markus Wenzel",
   title = "The {Isabelle/Isar} Reference Manual",
-  note = "Part of the Isabelle distribution, \url{http://isabelle.in.tum.de/doc/isar-ref.pdf}."
+  note = "Part of the Isabelle distribution, \url{https://isabelle.in.tum.de/doc/isar-ref.pdf}."
 }
 
 @book {Jacobson1985,