Clustering task fails to upload

One of the unit tests that I added for uploading clustering tasks is failing.
This is the task xml:

<oml:task_inputs xmlns:oml="http://openml.org/openml">
	<oml:task_type_id>5</oml:task_type_id>
	<oml:input name="source_data">77</oml:input>
</oml:task_inputs>

It fails with the following error:
openml.exceptions.OpenMLServerException: https://test.openml.org/api/v1/xml/task/ returned code 618: Task does not contain all required inputs.

I verified with the following xsd and it is validated:
https://github.com/openml/OpenML/blob/develop/openml_OS/views/pages/api_new/v1/xsd/openml.task.upload.xsd

One more question is why is target_feature required in clustering tasks:
https://www.openml.org/tt/5
It is empty in the clustering tasks that I checked in OpenML and it is not on the clustering task class in the openml-python connector:
https://github.com/openml/openml-python/blob/96db525e650e527b3283fb64bace8509639ce1f4/openml/tasks/task.py#L167

Start with the failing clustering-upload unit test and compare task type 5's required inputs against openml_OS/views/pages/api_new/v1/xsd/openml.task.upload.xsd. Then inspect openml-python's clustering task definition at task.py around line 167. Done means the reported clustering XML is accepted and its required-input behavior is consistent with the task definition.