diff --git a/scrapy/http/headers.py b/scrapy/http/headers.py
index fb612a55f..5efeda29c 100644
--- a/scrapy/http/headers.py
+++ b/scrapy/http/headers.py
@@ -19,12 +19,10 @@ class Headers(CaselessDict):
 
     def normvalue(self, value):
         """Headers must not be unicode"""
-        if isinstance(value, unicode):
-            value = value.encode(self.encoding)
-
-        if isinstance(value, (list, tuple)):
-            return list(value)
-        return [value]
+        if not hasattr(value, '__iter__'):
+            value = [value]
+        return [x.encode(self.encoding) if isinstance(x, unicode) else x \
+            for x in value]
 
     def __getitem__(self, key):
         try:
diff --git a/scrapy/tests/test_http_headers.py b/scrapy/tests/test_http_headers.py
index 871a987ca..7433ed830 100644
--- a/scrapy/tests/test_http_headers.py
+++ b/scrapy/tests/test_http_headers.py
@@ -1,5 +1,4 @@
 import unittest
-import weakref
 import copy
 
 from scrapy.http import Headers
@@ -34,6 +33,23 @@ class HeadersTest(unittest.TestCase):
         self.assertEqual(h.getlist('X-Forwarded-For'), hlist)
         assert h.getlist('X-Forwarded-For') is not hlist
 
+    def test_encode_utf8(self):
+        h = Headers({u'key': u'\xa3'}, encoding='utf-8')
+        key, val = dict(h).items()[0]
+        assert isinstance(key, str), key
+        assert isinstance(val[0], str), val[0]
+        self.assertEqual(val[0], '\xc2\xa3')
+
+    def test_encode_latin1(self):
+        h = Headers({u'key': u'\xa3'}, encoding='latin1')
+        key, val = dict(h).items()[0]
+        self.assertEqual(val[0], '\xa3')
+
+    def test_encode_multiple(self):
+        h = Headers({u'key': [u'\xa3']}, encoding='utf-8')
+        key, val = dict(h).items()[0]
+        self.assertEqual(val[0], '\xc2\xa3')
+
     def test_delete_and_contains(self):
         h = Headers()