diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 2313a6e41..82c3559ea 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -448,13 +448,18 @@ class Item(BaseModel):
             if list_append_re:
                 new_value = expression_attribute_values[list_append_re.group(2).strip()]
                 old_list_key = list_append_re.group(1)
-                # Get the existing value
-                old_list = self.attrs[old_list_key.split(".")[0]]
-                if "." in old_list_key:
-                    # Value is nested inside a map - find the appropriate child attr
-                    old_list = old_list.child_attr(
-                        ".".join(old_list_key.split(".")[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)]
                     )
+                else:
+                    old_list = self.attrs[old_list_key.split(".")[0]]
+                    if "." in old_list_key:
+                        # Value is nested inside a map - find the appropriate child attr
+                        old_list = old_list.child_attr(
+                            ".".join(old_list_key.split(".")[1:])
+                        )
                 if not old_list.is_list():
                     raise ParamValidationError
                 old_list.value.extend([DynamoType(v) for v in new_value["L"]])
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index ec01889ae..180f460c0 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -3609,6 +3609,31 @@ def test_update_supports_list_append_maps():
     )
 
 
+@mock_dynamodb2
+def test_update_supports_list_append_with_nested_if_not_exists_operation():
+    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", "nest1": {"nest2": {}}})
+    table.update_item(
+        Key={"Id": "item-id"},
+        UpdateExpression="SET nest1.nest2.event_history = list_append(if_not_exists(nest1.nest2.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", "nest1": {"nest2": {"event_history": ["some_value"]}}}
+    )
+
+
 @mock_dynamodb2
 def test_update_catches_invalid_list_append_operation():
     client = boto3.client("dynamodb", region_name="us-east-1")