diff --git a/message/externallib.php b/message/externallib.php
index 6373b8cf7ea..4239922df61 100644
--- a/message/externallib.php
+++ b/message/externallib.php
@@ -215,6 +215,9 @@ class core_message_external extends external_api {
                 //          We should have thrown exceptions as these errors prevent results to be returned.
                 // See http://docs.moodle.org/dev/Errors_handling_in_web_services#When_to_send_a_warning_on_the_server_side .
                 $resultmsg['msgid'] = -1;
+                if (!isset($errormessage)) { // Nobody has set a message error or thrown an exception, let's set it.
+                    $errormessage = get_string('messageundeliveredbynotificationsettings', 'error');
+                }
                 $resultmsg['errormessage'] = $errormessage;
             }