diff --git a/eopayment/paybox.py b/eopayment/paybox.py
index 472ac95..a00dd59 100644
--- a/eopayment/paybox.py
+++ b/eopayment/paybox.py
@@ -234,7 +234,7 @@ class Payment(PaymentCommon):
                 'name': 'shared_secret',
                 'caption': _('Secret partagé (clé HMAC)'),
                 'validation': lambda x: isinstance(x, str)
-                and all(a.lower() in '0123456789ABCDEF' for a in x),
+                and all(a.lower() in '0123456789abcdef' for a in x),
                 'required': True,
             },
             {