diff -r d943089af1c6 Lib/posixpath.py
--- a/Lib/posixpath.py	Mon Jun 30 04:04:52 2014 +0300
+++ b/Lib/posixpath.py	Sun Jun 29 23:24:59 2014 -0400
@@ -34,6 +34,7 @@
 defpath = ':/bin:/usr/bin'
 altsep = None
 devnull = '/dev/null'
+mixed_string_binary_error = "Can't mix strings and bytes in path components."
 
 def _get_sep(path):
     if isinstance(path, bytes):
@@ -87,8 +88,7 @@
                           for s in (a, ) + p)
         if valid_types:
             # Must have a mixture of text and binary data
-            raise TypeError("Can't mix strings and bytes in path "
-                            "components.") from None
+            raise TypeError(mixed_string_binary_error) from None
         raise
     return path
 
@@ -447,9 +447,15 @@
     if start is None:
         start = curdir
 
-    start_list = [x for x in abspath(start).split(sep) if x]
-    path_list = [x for x in abspath(path).split(sep) if x]
-
+    try:
+        start_list = [x for x in abspath(start).split(sep) if x]
+        path_list = [x for x in abspath(path).split(sep) if x]
+    except TypeError as e:
+        valid_types = all(isinstance(s, (str, bytes, bytearray))
+                          for s in [start, path])
+        if valid_types:
+            raise TypeError(mixed_string_binary_error) from None
+        raise
     # Work out how much of the filepath is shared by start and path.
     i = len(commonprefix([start_list, path_list]))
 
diff -r d943089af1c6 Lib/test/test_posixpath.py
--- a/Lib/test/test_posixpath.py	Mon Jun 30 04:04:52 2014 +0300
+++ b/Lib/test/test_posixpath.py	Sun Jun 29 23:24:59 2014 -0400
@@ -43,6 +43,15 @@
             support.unlink(support.TESTFN + suffix)
             safe_rmdir(support.TESTFN + suffix)
 
+    def check_error_msg(self, list_of_args, func_to_call, msg=
+                        "Can't mix strings and bytes in path components."):
+        """Check posixpath.join raises friendly TypeErrors."""
+        for args in (item for perm in list_of_args
+                          for item in itertools.permutations(perm)):
+            with self.assertRaises(TypeError) as cm:
+                func_to_call(*args)
+            self.assertEqual(msg, cm.exception.args[0])
+
     def test_join(self):
         self.assertEqual(posixpath.join("/foo", "bar", "/bar", "baz"),
                          "/bar/baz")
@@ -56,17 +65,7 @@
                          b"/foo/bar/baz")
         self.assertEqual(posixpath.join(b"/foo/", b"bar/", b"baz/"),
                          b"/foo/bar/baz/")
-
-        def check_error_msg(list_of_args, msg):
-            """Check posixpath.join raises friendly TypeErrors."""
-            for args in (item for perm in list_of_args
-                              for item in itertools.permutations(perm)):
-                with self.assertRaises(TypeError) as cm:
-                    posixpath.join(*args)
-                self.assertEqual(msg, cm.exception.args[0])
-
-        check_error_msg([[b'bytes', 'str'], [bytearray(b'bytes'), 'str']],
-                        "Can't mix strings and bytes in path components.")
+        self.check_error_msg([[b'bytes', 'str'], [bytearray(b'bytes'), 'str']], posixpath.join)
         # regression, see #15377
         with self.assertRaises(TypeError) as cm:
             posixpath.join(None, 'str')
@@ -487,6 +486,8 @@
         try:
             curdir = os.path.split(os.getcwd())[-1]
             self.assertRaises(ValueError, posixpath.relpath, "")
+            self.assertRaises(TypeError, 'a', b'a/b')
+            self.check_error_msg([[b'bytes', 'str'], [bytearray(b'bytes'), 'str']], posixpath.relpath)
             self.assertEqual(posixpath.relpath("a"), "a")
             self.assertEqual(posixpath.relpath(posixpath.abspath("a")), "a")
             self.assertEqual(posixpath.relpath("a/b"), "a/b")