--- a/mercurial/branchmap.py	Thu Mar 07 11:04:34 2024 +0100
+++ b/mercurial/branchmap.py	Fri Mar 08 15:06:54 2024 +0100
@@ -302,7 +302,8 @@
                 self._closednodes.add(cl.node(r))
             max_rev = max(max_rev, r)
         if max_rev < 0:
-            max_rev = None
+            msg = "running branchcache.update without revision to update"
+            raise error.ProgrammingError(msg)
 
         # Delay fetching the topological heads until they are needed.
         # A repository without non-continous branches can skip this part.