diff --git a/src/accounting/account/forms.py b/src/accounting/account/forms.py
index 24904b4..291aaee 100644
--- a/src/accounting/account/forms.py
+++ b/src/accounting/account/forms.py
@@ -57,10 +57,9 @@ class NoOffsetNominalAccount:
     """The validator to check nominal account is not to be offset."""
 
     def __call__(self, form: FlaskForm, field: BooleanField) -> None:
+        assert isinstance(form, AccountForm)
         if not field.data:
             return
-        if not isinstance(form, AccountForm):
-            return
         if form.base_code.data is None:
             return
         if form.base_code.data[0] not in {"1", "2"}:
diff --git a/src/accounting/currency/forms.py b/src/accounting/currency/forms.py
index b3dff4d..ae59cb9 100644
--- a/src/accounting/currency/forms.py
+++ b/src/accounting/currency/forms.py
@@ -32,10 +32,9 @@ class CodeUnique:
     """The validator to check if the code is unique."""
 
     def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, CurrencyForm)
         if field.data == "":
             return
-        if not isinstance(form, CurrencyForm):
-            return
         if form.obj_code is not None and form.obj_code == field.data:
             return
         if db.session.get(Currency, field.data) is not None:
diff --git a/src/accounting/transaction/forms/currency.py b/src/accounting/transaction/forms/currency.py
index 5f5c15c..36162db 100644
--- a/src/accounting/transaction/forms/currency.py
+++ b/src/accounting/transaction/forms/currency.py
@@ -61,8 +61,7 @@ class IsBalanced:
     entries are equal."""
 
     def __call__(self, form: FlaskForm, field: BooleanField) -> None:
-        if not isinstance(form, TransferCurrencyForm):
-            return
+        assert isinstance(form, TransferCurrencyForm)
         if len(form.debit) == 0 or len(form.credit) == 0:
             return
         if form.debit_total != form.credit_total: