2 files changed, 23 insertions, 2 deletions
diff --git a/src/mailman/handlers/subject_prefix.py b/src/mailman/handlers/subject_prefix.py
index 295ab039c..da971751e 100644
--- a/src/mailman/handlers/subject_prefix.py
+++ b/src/mailman/handlers/subject_prefix.py
@@ -69,7 +69,10 @@ def all_same_charset(mlist, msgdata, subject, prefix, prefix_pattern, ws):
     for chunk, charset in decode_header(subject.encode()):
         if charset is None:
             charset = 'us-ascii'
-        chunks.append(chunk.decode(charset))
+        if isinstance(chunk, str):
+            chunks.append(chunk)
+        else:
+            chunks.append(chunk.decode(charset))
         if charset != list_charset:
             return None
     subject_text = EMPTYSTRING.join(chunks)
@@ -111,7 +114,10 @@ def mixed_charsets(mlist, msgdata, subject, prefix, prefix_pattern, ws):
     chunk_text, chunk_charset = chunks[0]
     if chunk_charset is None:
         chunk_charset = 'us-ascii'
-    first_text = chunk_text.decode(chunk_charset)
+    if isinstance(chunk_text, str):
+        first_text = chunk_text
+    else:
+        first_text = chunk_text.decode(chunk_charset)
     first_text = re.sub(prefix_pattern, '', first_text).lstrip()
     rematch = re.match(RE_PATTERN, first_text, re.I)
     if rematch:
diff --git a/src/mailman/handlers/tests/test_subject_prefix.py b/src/mailman/handlers/tests/test_subject_prefix.py
index 41d0bf233..0203cabb1 100644
--- a/src/mailman/handlers/tests/test_subject_prefix.py
+++ b/src/mailman/handlers/tests/test_subject_prefix.py
@@ -22,7 +22,9 @@ import unittest
 from mailman.app.lifecycle import create_list
 from mailman.config import config
 from mailman.email.message import Message
+from mailman.interfaces.languages import ILanguageManager
 from mailman.testing.layers import ConfigLayer
+from zope.component import getUtility
 
 
 class TestSubjectPrefix(unittest.TestCase):
@@ -121,3 +123,16 @@ class TestSubjectPrefix(unittest.TestCase):
             subject.encode(),
             '[Test 456] Re: =?iso-2022-jp?b?GyRCJWEhPCVrJV4lcxsoQg==?=')
         self.assertEqual(str(subject), '[Test 456] Re: メールマン')
+
+    def test_decode_header_returns_string(self):
+        # Under some circumstances, email.header.decode_header() returns a
+        # string value.  Ensure we can handle that.
+        manager = getUtility(ILanguageManager)
+        manager.add('xx', 'iso-8859-1', 'Xlandia')
+        self._mlist.preferred_language = 'xx'
+        msg = Message()
+        msg['Subject'] = 'Plain text'
+        self._process(self._mlist, msg, {})
+        subject = msg['subject']
+        self.assertEqual(subject.encode(),
+                         '=?iso-8859-1?q?=5BTest=5D_?= Plain text')