diff -r 436b606ecfe8 Lib/test/test_urlparse.py
--- a/Lib/test/test_urlparse.py	Wed Sep 25 07:14:41 2013 -0700
+++ b/Lib/test/test_urlparse.py	Thu Sep 26 18:09:57 2013 +0800
@@ -679,6 +679,17 @@
             urllib.parse.urljoin("http://python.org", b"http://python.org")
         with self.assertRaisesRegex(TypeError, "Cannot mix str"):
             urllib.parse.urljoin(b"http://python.org", "http://python.org")
+        # issue 19094
+        with self.assertRaisesRegex(TypeError, "Cannot mix str"):
+            urllib.parse.urljoin("http://python.org", b"")
+        with self.assertRaisesRegex(TypeError, "Cannot mix str"):
+            urllib.parse.urljoin(b"http://python.org", "")
+        with self.assertRaisesRegex(TypeError, "Cannot mix str"):
+            urllib.parse.urljoin("http://python.org", [])
+        with self.assertRaisesRegex(TypeError, "Cannot mix bytes"):
+            urllib.parse.urljoin(b"http://python.org", [])
+        with self.assertRaisesRegex(TypeError, "Cannot mix bytes"):
+            urllib.parse.urljoin(bytearray(b"http://python.org"), [])
 
     def _check_result_type(self, str_type):
         num_args = len(str_type._fields)
diff -r 436b606ecfe8 Lib/urllib/parse.py
--- a/Lib/urllib/parse.py	Wed Sep 25 07:14:41 2013 -0700
+++ b/Lib/urllib/parse.py	Thu Sep 26 18:09:57 2013 +0800
@@ -405,6 +405,14 @@
 def urljoin(base, url, allow_fragments=True):
     """Join a base URL and a possibly relative URL to form an absolute
     interpretation of the latter."""
+    base_str = isinstance(base, str)
+    url_str = isinstance(url, str)
+    if base_str != url_str:
+        raise TypeError("Cannot mix str and non-str arguments")
+    base_bytes = any((isinstance(base, bytes), isinstance(base, bytearray)))
+    url_bytes = any((isinstance(url, bytes), isinstance(url, bytearray)))
+    if base_bytes != url_bytes:
+        raise TypeError("Cannot mix bytes and non-bytes arguments")
     if not base:
         return url
     if not url: