diff --git a/Lib/email/header.py b/Lib/email/header.py
index 6e03922..9e91dc2 100644
--- a/Lib/email/header.py
+++ b/Lib/email/header.py
@@ -92,6 +92,9 @@
                 if encoding == 'q':
                     dec = email.quoprimime.header_decode(encoded)
                 elif encoding == 'b':
+                    paderr = len(encoded) % 4   # Postel's law: add missing padding
+                    if paderr:
+                        encoded += '==='[:4 - paderr]
                     try:
                         dec = email.base64mime.decode(encoded)
                     except binascii.Error:
diff --git a/Lib/email/test/test_email.py b/Lib/email/test/test_email.py
index 067937a..e1754e9 100644
--- a/Lib/email/test/test_email.py
+++ b/Lib/email/test/test_email.py
@@ -1611,6 +1611,15 @@
                               ('rg', None), ('\xe5', 'iso-8859-1'),
                               ('sbord', None)])
 
+    def test_rfc2047_B_bad_padding(self):
+        s = '=?iso-8859-1?B?%s?='
+        data = [                                # only test complete bytes
+            ('dm==', 'v'), ('dm=', 'v'), ('dm', 'v'),
+            ('dmk=', 'vi'), ('dmk', 'vi')
+          ]
+        for q, a in data:
+            dh = decode_header(s % q)
+            self.assertEqual(dh, [(a, 'iso-8859-1')])
 
 
 # Test the MIMEMessage class
@@ -3064,7 +3073,7 @@
 
     def test_broken_base64_header(self):
         raises = self.assertRaises
-        s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3IQ?='
+        s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3I ?='
         raises(Errors.HeaderParseError, decode_header, s)
 
 
diff --git a/Lib/email/test/test_email_renamed.py b/Lib/email/test/test_email_renamed.py
index 976d892..481c40f 100644
--- a/Lib/email/test/test_email_renamed.py
+++ b/Lib/email/test/test_email_renamed.py
@@ -2958,7 +2958,7 @@
 
     def test_broken_base64_header(self):
         raises = self.assertRaises
-        s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3IQ?='
+        s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3I ?='
         raises(errors.HeaderParseError, decode_header, s)