diff -urN ../Mail-SpamAssassin-3.4.1.orig/lib/Mail/SpamAssassin/Message/Node.pm ./lib/Mail/SpamAssassin/Message/Node.pm
--- ../Mail-SpamAssassin-3.4.1.orig/lib/Mail/SpamAssassin/Message/Node.pm	2015-04-28 22:56:48.000000000 +0300
+++ ./lib/Mail/SpamAssassin/Message/Node.pm	2015-05-05 22:48:57.000000000 +0300
@@ -794,10 +794,10 @@
   $header_field_body =~ s/\n[ \t]+/\n /g;
   $header_field_body =~ s/\015?\012//gs;
 
-  if ($header_field_name =~
+  if ($header_field_name and ($header_field_name =~
        /^ (?: (?: Received | (?:Resent-)? (?: Message-ID | Date ) |
                   MIME-Version | References | In-Reply-To ) \z
-            | (?: List- | Content- ) ) /xsi ) {
+            | (?: List- | Content- ) ) /xsi )) {
     # Bug 6945: some header fields must not be processed for MIME encoding
 
   } else {