diff --git a/transformation_wrapper.php b/transformation_wrapper.php
index b879a7206f..25bc61bc7c 100644
--- a/transformation_wrapper.php
+++ b/transformation_wrapper.php
@@ -103,7 +103,7 @@ $response->getHeader()->sendHttpHeaders();
 if (isset($ct) && ! empty($ct)) {
     $mime_type = $ct;
 } else {
-    $mime_type = (isset($mime_map[$transform_key]['mimetype'])
+    $mime_type = (!empty($mime_map[$transform_key]['mimetype'])
         ? str_replace('_', '/', $mime_map[$transform_key]['mimetype'])
         : $default_ct)
     . (isset($mime_options['charset']) ? $mime_options['charset'] : '');