diff --git a/graphene_django/converter.py b/graphene_django/converter.py
index 8ae8b35..b9ee8a8 100644
--- a/graphene_django/converter.py
+++ b/graphene_django/converter.py
@@ -37,8 +37,8 @@ except ImportError:
     from graphql import assert_valid_name as assert_name
 from graphql.pyutils import register_description
 
-from .compat import ArrayField, HStoreField, RangeField, JSONField
-from .fields import DjangoListField, DjangoConnectionField
+from .compat import ArrayField, HStoreField, JSONField, RangeField
+from .fields import DjangoConnectionField, DjangoListField
 from .settings import graphene_settings
 from .utils.str_converters import to_const
 
diff --git a/graphene_django/forms/converter.py b/graphene_django/forms/converter.py
index 57e87b9..60996b4 100644
--- a/graphene_django/forms/converter.py
+++ b/graphene_django/forms/converter.py
@@ -27,8 +27,8 @@ def get_form_field_description(field):
 @singledispatch
 def convert_form_field(field):
     raise ImproperlyConfigured(
-        "Don't know how to convert the Django form field %s (%s) "
-        "to Graphene type" % (field, field.__class__)
+        f"Don't know how to convert the Django form field {field} ({field.__class__}) "
+        "to Graphene type"
     )