diff --git a/Lib/posixpath.py b/Lib/posixpath.py
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -74,13 +74,21 @@
     will be discarded."""
     sep = _get_sep(a)
     path = a
-    for b in p:
-        if b.startswith(sep):
-            path = b
-        elif not path or path.endswith(sep):
-            path +=  b
+    try:
+        for b in p:
+            if b.startswith(sep):
+                path = b
+            elif not path or path.endswith(sep):
+                path += b
+            else:
+                path += sep + b
+    except TypeError:
+        strs = [isinstance(s, str) for s in (a, ) + p]
+        if any(strs) and not all(strs):
+            raise TypeError('You can\'t mix strings and bytes in path '
+                            'components.') from None
         else:
-            path += sep + b
+            raise
     return path
 
 
diff --git a/Lib/test/test_posixpath.py b/Lib/test/test_posixpath.py
--- a/Lib/test/test_posixpath.py
+++ b/Lib/test/test_posixpath.py
@@ -58,6 +58,18 @@
 
         self.assertRaises(TypeError, posixpath.join, b"bytes", "str")
         self.assertRaises(TypeError, posixpath.join, "str", b"bytes")
+        try:
+            posixpath.join(b'bytes', 'str')
+        except TypeError as e:
+            self.assertIn('You can\'t mix strings and bytes', e.args[0])
+        try:
+            posixpath.join('str', b'bytes')
+        except TypeError as e:
+            self.assertIn('You can\'t mix strings and bytes', e.args[0])
+        try:
+            posixpath.join('str', bytearray(b'bytes'))
+        except TypeError as e:
+            self.assertIn('You can\'t mix strings and bytes', e.args[0])
 
     def test_split(self):
         self.assertEqual(posixpath.split("/foo/bar"), ("/foo", "bar"))