diff --git a/inginious/frontend/pages/register.py b/inginious/frontend/pages/register.py
index 4d51e838f..7c37ee284 100644
--- a/inginious/frontend/pages/register.py
+++ b/inginious/frontend/pages/register.py
@@ -67,7 +67,7 @@ def register_user(self, data):
         # Check input format
         if re.match(r"^[-_|~0-9A-Z]{4,}$", data["username"], re.IGNORECASE) is None:
             error = True
-            msg = _("Invalid username format. Username should only contain A-Z, 0-9, - _ ~ and at least be 4 characters long")
+            msg = _("Invalid username format. Username should only contain A-Z, a-z, 0-9, - _ ~ and at least be 4 characters long")
         elif email is None:
             error = True
             msg = _("Invalid email format.")