diff --git a/mercurial/dagutil.py b/mercurial/dagutil.py
--- a/mercurial/dagutil.py
+++ b/mercurial/dagutil.py
@@ -260,7 +260,7 @@ class inverserevlogdag(revlogbaseddag, g
             if isroot:
                 roots.append(rev)
             rev -= 1
-        self._walkfrom = rev - 1
+        self._walkfrom = rev
 
     def _getheads(self):
         self._walkto(nullrev)
diff --git a/tests/test-setdiscovery.t b/tests/test-setdiscovery.t
--- a/tests/test-setdiscovery.t
+++ b/tests/test-setdiscovery.t
@@ -260,7 +260,7 @@ Both many new on top of long history:
   query 2; still undecided: 1029, sample size is: 11
   sampling from both directions
   searching: 3 queries
-  query 3; still undecided: 16, sample size is: 16
+  query 3; still undecided: 15, sample size is: 15
   3 total queries
   common heads: 7ead0cba2838