diff --git a/dkim/tests/test_util.py b/dkim/tests/test_util.py
index 1ee6ccc..c9b62e1 100644
--- a/dkim/tests/test_util.py
+++ b/dkim/tests/test_util.py
@@ -30,38 +30,38 @@ class TestParseTagValue(unittest.TestCase):
 
     def test_single(self):
         self.assertEqual(
-            {'foo': 'bar'},
-            parse_tag_value('foo=bar'))
+            {b'foo': b'bar'},
+            parse_tag_value(b'foo=bar'))
 
     def test_trailing_separator_ignored(self):
         self.assertEqual(
-            {'foo': 'bar'},
-            parse_tag_value('foo=bar;'))
+            {b'foo': b'bar'},
+            parse_tag_value(b'foo=bar;'))
 
     def test_multiple(self):
         self.assertEqual(
-            {'foo': 'bar', 'baz': 'foo'},
-            parse_tag_value('foo=bar;baz=foo'))
+            {b'foo': b'bar', b'baz': b'foo'},
+            parse_tag_value(b'foo=bar;baz=foo'))
 
     def test_value_with_equals(self):
         self.assertEqual(
-            {'foo': 'bar', 'baz': 'foo=bar'},
-            parse_tag_value('foo=bar;baz=foo=bar'))
+            {b'foo': b'bar', b'baz': b'foo=bar'},
+            parse_tag_value(b'foo=bar;baz=foo=bar'))
 
     def test_whitespace_is_stripped(self):
         self.assertEqual(
-            {'foo': 'bar', 'baz': 'f oo=bar'},
-            parse_tag_value('   foo  \t= bar;\tbaz=  f oo=bar  '))
+            {b'foo': b'bar', b'baz': b'f oo=bar'},
+            parse_tag_value(b'   foo  \t= bar;\tbaz=  f oo=bar  '))
 
     def test_missing_value_is_an_error(self):
         self.assertRaisesRegexp(
             InvalidTagSpec, 'baz',
-            parse_tag_value, 'foo=bar;baz')
+            parse_tag_value, b'foo=bar;baz')
 
     def test_duplicate_tag_is_an_error(self):
         self.assertRaisesRegexp(
             DuplicateTag, 'foo',
-            parse_tag_value, 'foo=bar;foo=baz')
+            parse_tag_value, b'foo=bar;foo=baz')
 
 
 def test_suite():
diff --git a/dkim/util.py b/dkim/util.py
index 0cdda93..13c56d4 100644
--- a/dkim/util.py
+++ b/dkim/util.py
@@ -55,13 +55,13 @@ def parse_tag_value(tag_list):
     @param tag_list: A string containing a DKIM Tag=Value list.
     """
     tags = {}
-    tag_specs = tag_list.split(';')
+    tag_specs = tag_list.split(b';')
     # Trailing semicolons are valid.
     if not tag_specs[-1]:
         tag_specs.pop()
     for tag_spec in tag_specs:
         try:
-            key, value = tag_spec.split('=', 1)
+            key, value = tag_spec.split(b'=', 1)
         except ValueError:
             raise InvalidTagSpec(tag_spec)
         if key.strip() in tags: