diff --git a/program/include/rcube_shared.inc b/program/include/rcube_shared.inc
index 99d8b16df..e93e9aefe 100644
--- a/program/include/rcube_shared.inc
+++ b/program/include/rcube_shared.inc
@@ -578,7 +578,9 @@ function json_serialize($input)
 {
   $input = rc_utf8_clean($input);
 
-  return json_encode($input);
+  // sometimes even using rc_utf8_clean() the input contains invalid UTF-8 sequences
+  // that's why we have @ here
+  return @json_encode($input);
 }