diff -r faa015417348 -r fc72beec2a1a mercurial/revlog.py
--- a/mercurial/revlog.py	Wed Mar 07 11:10:22 2018 +0100
+++ b/mercurial/revlog.py	Sat Apr 21 17:27:57 2018 +0200
@@ -877,10 +877,11 @@
             return base
 
         index = self.index
-        base = index[rev][3]
-        while base != rev:
-            rev = base
-            base = index[rev][3]
+        iterrev = rev
+        base = index[iterrev][3]
+        while base != iterrev:
+            iterrev = base
+            base = index[iterrev][3]
 
         self._chainbasecache[rev] = base
         return base