diff --git a/message/externallib.php b/message/externallib.php
index c0046d2b1c9..a9ff688a7b5 100644
--- a/message/externallib.php
+++ b/message/externallib.php
@@ -1708,7 +1708,7 @@ class core_message_external extends external_api {
         } else {
             // We have to throw an exception here because the external functions annoyingly
             // don't accept null to be returned for a single structure.
-            throw new \moodle_exception('Conversation does not exist');
+            throw new \moodle_exception('conversationdoesntexist');
         }
     }
 
@@ -1813,7 +1813,7 @@ class core_message_external extends external_api {
         } else {
             // We have to throw an exception here because the external functions annoyingly
             // don't accept null to be returned for a single structure.
-            throw new \moodle_exception('Conversation does not exist');
+            throw new \moodle_exception('conversationdoesntexist');
         }
     }