diff --git a/c3l_membership/forms.py b/c3l_membership/forms.py
--- a/c3l_membership/forms.py
+++ b/c3l_membership/forms.py
@@ -1,8 +1,11 @@
+from typing import Optional as tOptional
+
 from flask_babel import lazy_gettext
 from flask_wtf import FlaskForm
 from wtforms import (
     BooleanField,
     DateField,
+    Field,
     RadioField,
     StringField,
     SubmitField,
@@ -22,11 +25,11 @@ class NotEqualTo:
         more helpful error.
     """
 
-    def __init__(self, fieldname, message=None):
+    def __init__(self, fieldname: str, message: tOptional[str] = None) -> None:
         self.fieldname = fieldname
         self.message = message
 
-    def __call__(self, form, field):
+    def __call__(self, form: FlaskForm, field: Field) -> None:
         try:
             other = form[self.fieldname]
         except KeyError as exc: