diff --git a/lancaster-university-harvard.csl b/lancaster-university-harvard.csl
index 2877d583497..3c597b4306a 100644
--- a/lancaster-university-harvard.csl
+++ b/lancaster-university-harvard.csl
@@ -436,6 +436,7 @@
   <bibliography hanging-indent="false" et-al-min="3" et-al-use-first="1">
     <sort>
       <key macro="author"/>
+      <key variable="citation-number"/>
       <key variable="title"/>
     </sort>
     <layout>