diff --git a/lflex_celcat_survival/fetch.py b/lflex_celcat_survival/fetch.py
index a6bbac0ce761e3e7714f87e908baf2235df14b56..b1deb1d53706f62c24d1fdc041f201290ccc3b29 100644
--- a/lflex_celcat_survival/fetch.py
+++ b/lflex_celcat_survival/fetch.py
@@ -2,6 +2,7 @@ import logging
 import requests
 
 def do_celcat_calendar_request(min_date, max_date, module_apogee_codes, session, url='https://edt.univ-tlse3.fr/calendar/Home/GetCalendarData'):
+    assert min_date < max_date, f"min_date (value={min_date}) should be strictly greater than max_date (value={max_date})"
     headers = {"Content-Type": "application/x-www-form-urlencoded; charset=UTF-8"}
     fields = [
         f'start={min_date}',