diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 88f750775..8e5a61755 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -450,9 +450,9 @@ class Item(BaseModel):
                 old_list_key = list_append_re.group(1)
                 # old_key could be a function itself (if_not_exists)
                 if old_list_key.startswith("if_not_exists"):
-                    old_list = DynamoType(
-                        expression_attribute_values[self._get_default(old_list_key)]
-                    )
+                    old_list = self._get_default(old_list_key)
+                    if not isinstance(old_list, DynamoType):
+                        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 180f460c0..428b58f81 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -3634,6 +3634,31 @@ def test_update_supports_list_append_with_nested_if_not_exists_operation():
     )
 
 
+@mock_dynamodb2
+def test_update_supports_list_append_with_nested_if_not_exists_operation_and_property_already_exists():
+    dynamo = boto3.resource("dynamodb", region_name="us-west-1")
+    table_name = "test"
+
+    dynamo.create_table(
+        TableName=table_name,
+        AttributeDefinitions=[{"AttributeName": "Id", "AttributeType": "S"}],
+        KeySchema=[{"AttributeName": "Id", "KeyType": "HASH"}],
+        ProvisionedThroughput={"ReadCapacityUnits": 20, "WriteCapacityUnits": 20},
+    )
+
+    table = dynamo.Table(table_name)
+
+    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)",
+        ExpressionAttributeValues={":empty_list": [], ":new_value": ["some_value"]},
+    )
+    table.get_item(Key={"Id": "item-id"})["Item"].should.equal(
+        {"Id": "item-id", "event_history": ["other_value", "some_value"]}
+    )
+
+
 @mock_dynamodb2
 def test_update_catches_invalid_list_append_operation():
     client = boto3.client("dynamodb", region_name="us-east-1")