#1874: detect invalid multipart CTE and report it as a defect.
diff --git a/Lib/email/errors.py b/Lib/email/errors.py
index c2ea7d4..c04deb4 100644
--- a/Lib/email/errors.py
+++ b/Lib/email/errors.py
@@ -55,3 +55,6 @@
 
 class MultipartInvariantViolationDefect(MessageDefect):
     """A message claimed to be a multipart but no subparts were found."""
+
+class InvalidMultipartContentTransferEncodingDefect(MessageDefect):
+    """An invalid content transfer encoding was set on the multipart itself."""