diff --git a/ssoauth/auth_utils.py b/ssoauth/auth_utils.py
index 383c229d56782ddf0902edf20f4dc2b2ccfcb7f9..cd86bf6e93bbb408f96bdaafcb0da43c9773b996 100644
--- a/ssoauth/auth_utils.py
+++ b/ssoauth/auth_utils.py
@@ -12,7 +12,7 @@ import importlib
 
 def _validate_username(username):
     assert isinstance(username, str)
-    assert username.islower()
+    assert username == username.lower()  # because .islower() returns False when no letters present
 
 
 def get_user(uuid=None, username=None):