diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -1621,7 +1621,7 @@ def parentspec(repo, subset, x, n, order
             ps.add(cl.parentrevs(r)[0])
         elif n == 2:
             parents = cl.parentrevs(r)
-            if len(parents) > 1:
+            if parents[1] != node.nullrev:
                 ps.add(parents[1])
     return subset & ps
 
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -2750,6 +2750,7 @@ parentrevspec
   5
   $ log 'merge()^2'
   4
+  $ log '(not merge())^2'
   $ log 'merge()^^'
   3
   $ log 'merge()^1^'