diff --git a/gclient_scm.py b/gclient_scm.py
index 9196f680c..3b1b59f7a 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -495,7 +495,7 @@ class GitWrapper(SCMWrapper):
         self.Print('Will cherrypick %r .. %r on top of %r.' % (
             target_rev, pr, base_rev))
         try:
-          if scm.GIT.IsAncestor(self.checkout_path, pr, target_rev):
+          if scm.GIT.IsAncestor(pr, target_rev, cwd=self.checkout_path):
             if len(patch_revs_to_process) > 1:
               # If there are multiple patch_revs_to_process then we do not want
               # want to invalidate a previous patch so throw an error.
diff --git a/git_cl.py b/git_cl.py
index c6a9bb300..b1eafde67 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -4668,12 +4668,12 @@ def _UploadAllPrecheck(options, orig_args):
     # Case 4: If upstream's last_upload < cl.base_commit we are
     # uploading cl and upstream_cl.
     # Continue up the tree to check other branch relations.
-    if scm.GIT.IsAncestor(None, upstream_last_upload, base_commit):
+    if scm.GIT.IsAncestor(upstream_last_upload, base_commit):
       continue
 
     # Case 5: If cl.base_commit < upstream's last_upload the user
     # must rebase before uploading.
-    if scm.GIT.IsAncestor(None, base_commit, upstream_last_upload):
+    if scm.GIT.IsAncestor(base_commit, upstream_last_upload):
       DieWithError(
           'At least one branch in the stack has diverged from its upstream '
           'branch and does not contain its upstream\'s last upload.\n'
diff --git a/scm.py b/scm.py
index 8368bd010..4013a7625 100644
--- a/scm.py
+++ b/scm.py
@@ -314,7 +314,8 @@ class GIT(object):
     return upstream_branch
 
   @staticmethod
-  def IsAncestor(cwd, maybe_ancestor, ref):
+  def IsAncestor(maybe_ancestor, ref, cwd=None):
+    # type: (string, string, Optional[string]) -> bool
     """Verifies if |maybe_ancestor| is an ancestor of |ref|."""
     try:
       GIT.Capture(['merge-base', '--is-ancestor', maybe_ancestor, ref], cwd=cwd)
diff --git a/tests/scm_unittest.py b/tests/scm_unittest.py
index 0eb5b675e..52d560d3c 100755
--- a/tests/scm_unittest.py
+++ b/tests/scm_unittest.py
@@ -159,12 +159,15 @@ class RealGitTest(fake_repos.FakeReposTestBase):
     self.assertTrue(scm.GIT.IsValidRevision(cwd=self.cwd, rev='HEAD'))
 
   def testIsAncestor(self):
-    self.assertTrue(scm.GIT.IsAncestor(
-        self.cwd, self.githash('repo_1', 1), self.githash('repo_1', 2)))
-    self.assertFalse(scm.GIT.IsAncestor(
-        self.cwd, self.githash('repo_1', 2), self.githash('repo_1', 1)))
-    self.assertFalse(scm.GIT.IsAncestor(
-        self.cwd, self.githash('repo_1', 1), 'zebra'))
+    self.assertTrue(
+        scm.GIT.IsAncestor(self.githash('repo_1', 1),
+                           self.githash('repo_1', 2),
+                           cwd=self.cwd))
+    self.assertFalse(
+        scm.GIT.IsAncestor(self.githash('repo_1', 2),
+                           self.githash('repo_1', 1),
+                           cwd=self.cwd))
+    self.assertFalse(scm.GIT.IsAncestor(self.githash('repo_1', 1), 'zebra'))
 
   def testGetAllFiles(self):
     self.assertEqual(['DEPS','origin'], scm.GIT.GetAllFiles(self.cwd))