diff -r 02850baddadd -r 93313f66b69b mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Mon Jul 09 15:33:49 2018 -0700
+++ b/mercurial/debugcommands.py	Thu Jun 21 18:19:57 2018 +0200
@@ -678,8 +678,15 @@
         except IndexError:
             prevrev = -1
 
-        chainratio = float(chainsize) / float(uncomp)
-        extraratio = float(extradist) / float(chainsize)
+        if uncomp != 0:
+            chainratio = float(chainsize) / float(uncomp)
+        else:
+            chainratio = chainsize
+
+        if chainsize != 0:
+            extraratio = float(extradist) / float(chainsize)
+        else:
+            extraratio = extradist
 
         fm.startitem()
         fm.write('rev chainid chainlen prevrev deltatype compsize '