Index: modules/mimemail_compress/mimemail_compress.inc
===================================================================
--- modules/mimemail_compress/mimemail_compress.inc	(revision 1369)
+++ modules/mimemail_compress/mimemail_compress.inc	(working copy)
@@ -62,7 +62,12 @@
     }
 
     $encoding = mb_detect_encoding($body);
-    $body = mb_convert_encoding($body, 'HTML-ENTITIES', $encoding);
+    if ($encoding) {
+      $body = mb_convert_encoding($body, 'HTML-ENTITIES', $encoding);
+    }
+    else {
+      $body = mb_convert_encoding($body, 'HTML-ENTITIES', 'auto');
+    }
 
     $err = error_reporting(0);
     $doc = new DOMDocument;