diff --git a/src/utils/utf8.php b/src/utils/utf8.php
--- a/src/utils/utf8.php
+++ b/src/utils/utf8.php
@@ -712,7 +712,16 @@
         'mbstring'));
   }
 
-  $result = @mb_convert_encoding($string, $to_encoding, $from_encoding);
+  try {
+    $result = @mb_convert_encoding($string, $to_encoding, $from_encoding);
+  } catch (ValueError $ex) {
+    throw new Exception(
+      pht(
+        "String conversion from encoding '%s' to encoding '%s' failed: %s",
+        $from_encoding,
+        $to_encoding,
+        $ex));
+  }
 
   if ($result === false) {
     $message = error_get_last();