--- a/mercurial/revset.py	Sun Jun 18 11:57:28 2017 +0900
+++ b/mercurial/revset.py	Sun Jun 18 12:06:22 2017 +0900
@@ -250,7 +250,11 @@
     """Changesets that are ancestors of changesets in set, including the
     given changesets themselves.
     """
-    return _ancestors(repo, subset, x)
+    args = getargsdict(x, 'ancestors', 'set')
+    if 'set' not in args:
+        # i18n: "ancestors" is a keyword
+        raise error.ParseError(_('ancestors takes at least 1 argument'))
+    return _ancestors(repo, subset, args['set'])
 
 @predicate('_firstancestors', safe=True)
 def _firstancestors(repo, subset, x):
@@ -596,7 +600,11 @@
     """Changesets which are descendants of changesets in set, including the
     given changesets themselves.
     """
-    return _descendants(repo, subset, x)
+    args = getargsdict(x, 'descendants', 'set')
+    if 'set' not in args:
+        # i18n: "descendants" is a keyword
+        raise error.ParseError(_('descendants takes at least 1 argument'))
+    return _descendants(repo, subset, args['set'])
 
 @predicate('_firstdescendants', safe=True)
 def _firstdescendants(repo, subset, x):

--- a/tests/test-revset.t	Sun Jun 18 11:57:28 2017 +0900
+++ b/tests/test-revset.t	Sun Jun 18 12:06:22 2017 +0900
@@ -2996,8 +2996,7 @@
         ('symbol', '1'))
       any)
     define)
-  hg: parse error: can't use a list in this context
-  (see hg help "revsets.x or y")
+  hg: parse error: ancestors takes at most 1 positional arguments
   [255]
   $ hg debugrevspec -p optimized 'ancestors(6, 1) - ancestors(4)'
   * optimized:
@@ -3013,8 +3012,7 @@
       ('symbol', '4')
       any)
     define)
-  hg: parse error: can't use a list in this context
-  (see hg help "revsets.x or y")
+  hg: parse error: ancestors takes at most 1 positional arguments
   [255]
 
 optimization disabled if keyword arguments passed (because we're too lazy
@@ -3036,8 +3034,9 @@
         ('symbol', '4'))
       any)
     define)
-  hg: parse error: can't use a key-value pair in this context
-  [255]
+  3
+  5
+  6
 
 invalid function call should not be optimized to only()