diff --git a/app.py b/app.py
index aa5e8ef..e2cc6e6 100755
--- a/app.py
+++ b/app.py
@@ -6,7 +6,7 @@
 from configparser import ConfigParser
 from ldap3 import Connection, LDAPBindError, LDAPInvalidCredentialsResult, Server
 from ldap3 import AUTH_SIMPLE, SUBTREE
-from ldap3.core.exceptions import LDAPConstraintViolationResult
+from ldap3.core.exceptions import LDAPConstraintViolationResult, LDAPUserNameIsMandatoryError
 import os
 from os import path
 
@@ -62,7 +62,7 @@
         else:
             change_password_ldap(*args)
 
-    except (LDAPBindError, LDAPInvalidCredentialsResult):
+    except (LDAPBindError, LDAPInvalidCredentialsResult, LDAPUserNameIsMandatoryError):
         raise Error('Username or password is incorrect!')
 
     except LDAPConstraintViolationResult as e:
@@ -75,6 +75,7 @@
     with connect_ldap() as c:
         user_dn = find_user_dn(c, username)
 
+    # Note: raises LDAPUserNameIsMandatoryError when user_dn is None.
     with connect_ldap(authentication=AUTH_SIMPLE, user=user_dn, password=old_pass) as c:
         c.bind()
         c.extend.standard.modify_password(user_dn, old_pass, new_pass)