diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index ae670c2353c858a..8922bdcd85a2f77 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -186,8 +186,9 @@ def _write(self, msg):
         # If we munged the cte, copy the message again and re-fix the CTE.
         if munge_cte:
             msg = deepcopy(msg)
-            msg.replace_header('content-transfer-encoding', munge_cte[0])
             msg.replace_header('content-type', munge_cte[1])
+            if msg.get('content-transfer-encoding') is not None:
+                msg.replace_header('content-transfer-encoding', munge_cte[0])
         # Write the headers.  First we see if the message object wants to
         # handle that itself.  If not, we'll do it generically.
         meth = getattr(msg, '_write_headers', None)
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 621754cf753daa2..66154b5863d66fb 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -311,6 +311,34 @@ def test_as_string_policy(self):
         g.flatten(msg)
         self.assertEqual(fullrepr, s.getvalue())
 
+    def test_nonascii_as_string_without_cte(self):
+        m = textwrap.dedent("""\
+            MIME-Version: 1.0
+            Content-Type: multipart/alternative;
+             boundary="----5F0D91A9C8C24D91AF71E1D7F2F7877E"
+
+            ------5F0D91A9C8C24D91AF71E1D7F2F7877E
+            Content-type: text/plain; charset=utf-8
+
+            Test if UTF-8 messages with no Content-Transfer-Encoding set can be as_string'd:
+            Föö bär
+            ------5F0D91A9C8C24D91AF71E1D7F2F7877E--
+
+            """)
+        msg = email.message_from_string(m)
+        msg.as_string()
+
+    def test_nonascii_as_string_without_content_type_and_cte(self):
+        m = textwrap.dedent("""\
+            MIME-Version: 1.0
+
+            Test if UTF-8 messages with no Content-Type nor Content-Transfer-Encoding set can be as_string'd:
+            Föö bär
+
+            """)
+        msg = email.message_from_string(m)
+        msg.as_string()
+
     def test_as_bytes(self):
         msg = self._msgobj('msg_01.txt')
         with openfile('msg_01.txt') as fp:
diff --git a/Misc/NEWS.d/next/Library/2018-01-25-21-17-32.bpo-27321.AoxcPm.rst b/Misc/NEWS.d/next/Library/2018-01-25-21-17-32.bpo-27321.AoxcPm.rst
new file mode 100644
index 000000000000000..849326466e7457d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-01-25-21-17-32.bpo-27321.AoxcPm.rst
@@ -0,0 +1,2 @@
+Only replace the munged Content-Type-Encoding header when it actually exists
+while flattening an email.