diff --git a/mercurial/revlogutils/deltas.py b/mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py
+++ b/mercurial/revlogutils/deltas.py
@@ -257,13 +257,13 @@ def _slicechunktodensity(revlog, revs, t
         return
 
     deltachainspan = segmentspan(revlog, revs)
-    chainpayload = sum(length(r) for r in revs)
 
     if deltachainspan < mingapsize:
         yield revs
         return
 
     readdata = deltachainspan
+    chainpayload = sum(length(r) for r in revs)
 
     if deltachainspan:
         density = chainpayload / float(deltachainspan)