diff --git a/tests/test_source_maarch.py b/tests/test_source_maarch.py
index d01c924..8a2461b 100644
--- a/tests/test_source_maarch.py
+++ b/tests/test_source_maarch.py
@@ -236,7 +236,7 @@ def test_feed(settings, app, maarch, wcs, user):
         'status': 'GRCSENT',
         'externalInfos': [
             {
-                'external_id': 1,
+                'external_id': '1',
                 'external_link': 'http://wcs.example.net/slug-1/1',
                 'res_id': 1,
             }
diff --git a/welco/sources/mail/utils.py b/welco/sources/mail/utils.py
index d8c087f..953e73a 100644
--- a/welco/sources/mail/utils.py
+++ b/welco/sources/mail/utils.py
@@ -46,7 +46,7 @@ class WelcoMaarchCourrier(MaarchCourrier):
 
     def set_grc_sent_status(self, mail_pk, formdata_id, formdata_url_backoffice):
         mail = self.Courrier(self, pk=mail_pk)
-        mail.external_id = formdata_id
+        mail.external_id = str(formdata_id)
         mail.external_link = formdata_url_backoffice
         self.update_external_infos([mail], self.grc_send_status)