diff --git a/tests/trychange_unittest.py b/tests/trychange_unittest.py
index 3e8887e0b..8d102eac7 100755
--- a/tests/trychange_unittest.py
+++ b/tests/trychange_unittest.py
@@ -22,6 +22,7 @@ class TryChangeTestsBase(SuperMoxTestBase):
   def setUp(self):
     SuperMoxTestBase.setUp(self)
     self.mox.StubOutWithMock(subprocess2, 'communicate')
+    self.mox.StubOutWithMock(trychange, 'RunGit')
     self.mox.StubOutWithMock(trychange.scm.GIT, 'Capture')
     self.mox.StubOutWithMock(trychange.scm.GIT, 'GenerateDiff')
     self.mox.StubOutWithMock(trychange.scm.GIT, 'GetCheckoutRoot')
@@ -46,14 +47,12 @@ class TryChangeUnittest(TryChangeTestsBase):
   """General trychange.py tests."""
   def testMembersChanged(self):
     members = [
-      'EPILOG', 'Escape', 'GIT', 'GuessVCS', 'GetMungedDiff', 'HELP_STRING',
-      'InvalidScript', 'NoTryServerAccess', 'PrintSuccess', 'SCM', 'SVN',
-      'TryChange', 'USAGE',
-      'breakpad', 'datetime', 'errno', 'fix_encoding', 'gcl', 'gclient_utils',
-      'getpass', 'gen_parser',
-      'json', 'logging', 'optparse', 'os', 'posixpath', 're', 'scm', 'shutil',
-      'subprocess2', 'sys', 'tempfile', 'urllib',
-    ]
+      'DieWithError', 'EPILOG', 'Escape', 'GIT', 'GetMungedDiff', 'GuessVCS',
+      'HELP_STRING', 'InvalidScript', 'NoTryServerAccess', 'PrintSuccess',
+      'RunCommand', 'RunGit', 'SCM', 'SVN', 'TryChange', 'USAGE', 'breakpad',
+      'datetime', 'errno', 'fix_encoding', 'gcl', 'gclient_utils', 'gen_parser',
+      'getpass', 'json', 'logging', 'optparse', 'os', 'posixpath', 're', 'scm',
+      'shutil', 'subprocess2', 'sys', 'tempfile', 'urllib']
     # If this test fails, you should add the relevant test.
     self.compareMembers(trychange, members)
 
@@ -138,6 +137,7 @@ class GITUnittest(TryChangeTestsBase):
     trychange.os.path.abspath(self.fake_root).AndReturn(self.fake_root)
     trychange.scm.GIT.GetCheckoutRoot(self.fake_root).AndReturn(self.fake_root)
     trychange.scm.GIT.GetUpstreamBranch(self.fake_root).AndReturn('somewhere')
+    trychange.RunGit(['diff-index', 'HEAD'])
     trychange.scm.GIT.GenerateDiff(self.fake_root,
                                    full_move=True,
                                    files=['foo.txt', 'bar.txt'],
diff --git a/trychange.py b/trychange.py
index ceeeecea9..042b9391c 100755
--- a/trychange.py
+++ b/trychange.py
@@ -65,6 +65,28 @@ Examples:
             -f include/b.h
 """
 
+
+def DieWithError(message):
+  print >> sys.stderr, message
+  sys.exit(1)
+
+
+def RunCommand(args, error_ok=False, error_message=None, **kwargs):
+  try:
+    return subprocess2.check_output(args, shell=False, **kwargs)
+  except subprocess2.CalledProcessError, e:
+    if not error_ok:
+      DieWithError(
+          'Command "%s" failed.\n%s' % (
+            ' '.join(args), error_message or e.stdout or ''))
+    return e.stdout
+
+
+def RunGit(args, **kwargs):
+  """Returns stdout."""
+  return RunCommand(['git'] + args, **kwargs)
+
+
 class InvalidScript(Exception):
   def __str__(self):
     return self.args[0] + '\n' + HELP_STRING
@@ -272,6 +294,9 @@ class GIT(SCM):
         self.diff_against)
 
   def GenerateDiff(self):
+    if RunGit(['diff-index', 'HEAD']):
+      print 'Cannot try with a dirty tree.  You must commit locally first.'
+      return None
     return scm.GIT.GenerateDiff(
         self.checkout_root,
         files=self.files,
@@ -750,7 +775,10 @@ def TryChange(argv,
       root = checkouts[0].checkout_root
       diffs = []
       for checkout in checkouts:
-        diff = checkout.GenerateDiff().splitlines(True)
+        raw_diff = checkout.GenerateDiff()
+        if not raw_diff:
+          return 1
+        diff = raw_diff.splitlines(True)
         path_diff = gclient_utils.PathDifference(root, checkout.checkout_root)
         # Munge it.
         diffs.extend(GetMungedDiff(path_diff, diff)[0])