diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 7d995486..8e5a6175 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -452,9 +452,7 @@ class Item(BaseModel):
                 if old_list_key.startswith("if_not_exists"):
                     old_list = self._get_default(old_list_key)
                     if not isinstance(old_list, DynamoType):
-                        old_list = DynamoType(
-                            expression_attribute_values[old_list]
-                        )
+                        old_list = DynamoType(expression_attribute_values[old_list])
                 else:
                     old_list = self.attrs[old_list_key.split(".")[0]]
                     if "." in old_list_key:
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index d36fdc7f..428b58f8 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -3648,7 +3648,7 @@ def test_update_supports_list_append_with_nested_if_not_exists_operation_and_pro
 
     table = dynamo.Table(table_name)
 
-    table.put_item(Item={"Id": "item-id", "event_history":["other_value"]})
+    table.put_item(Item={"Id": "item-id", "event_history": ["other_value"]})
     table.update_item(
         Key={"Id": "item-id"},
         UpdateExpression="SET event_history = list_append(if_not_exists(event_history, :empty_list), :new_value)",