diff --git a/itsdangerous.py b/itsdangerous.py
index 05362da..3a6fc8f 100644
--- a/itsdangerous.py
+++ b/itsdangerous.py
@@ -349,7 +349,9 @@ def get_signature(self, value):
 
     def sign(self, value):
         """Signs the given string."""
-        return value + want_bytes(self.sep) + self.get_signature(value)
+        return want_bytes(value) + \
+            want_bytes(self.sep) + \
+            self.get_signature(value)
 
     def verify_signature(self, value, sig):
         """Verifies the signature for the given value."""
diff --git a/tests.py b/tests.py
index 6fbef79..11abcd9 100755
--- a/tests.py
+++ b/tests.py
@@ -28,6 +28,16 @@ def test_want_bytes(self):
         self.assertEqual(want_bytes(u"foobar"), b"foobar")
 
 
+class SignerTestCase(unittest.TestCase):
+    signer_class = idmod.Signer
+    def make_signer(self, *args, **kwargs):
+        return self.signer_class(*args, **kwargs)
+
+    def test_sign(self):
+        s = self.make_signer('secret-key')
+        assert isinstance(s.sign('my string'), bytes)
+
+
 class SerializerTestCase(unittest.TestCase):
     serializer_class = idmod.Serializer