diff --git a/ldap_auth_provider.py b/ldap_auth_provider.py
index 9398350..dcefe15 100644
--- a/ldap_auth_provider.py
+++ b/ldap_auth_provider.py
@@ -133,6 +133,8 @@ async def check_auth(
             except ActiveDirectoryUPNException:
                 return None
 
+        localpart = localpart.lower()
+
         try:
             server = self._get_server()
             logger.debug("Attempting LDAP connection with %s", self.ldap_uris)
diff --git a/tests/test_simple.py b/tests/test_simple.py
index 0ffaf7f..6e570b0 100644
--- a/tests/test_simple.py
+++ b/tests/test_simple.py
@@ -95,6 +95,15 @@ def test_no_pwd(self):
         )
         self.assertFalse(result)
 
+    @defer.inlineCallbacks
+    def test_uppercase_username(self):
+        result = yield defer.ensureDeferred(
+            self.auth_provider.check_auth(
+                "BOB", "m.login.password", {"password": "secret"}
+            )
+        )
+        self.assertEqual(result, "@bob:test")
+
 
 class LdapSearchTestCase(unittest.TestCase):
     @defer.inlineCallbacks