How to make subset of data based on index label range along specified dimension?

Hello! I am working on plotting subsets of my data (humidity variation in different height ranges). 

I hope to obtain data intervals of 0-4 km, 4-8 km and 8-12 km in the height coordinate.

For that purpose, I have used the cube.select function to define my subsets:

"height_range_1 = ct.cube.select(data, height=[0, 4000],vertical='height').height.values"

however, I receive the following error:

Traceback (most recent call last):
File "/opt/cdstoolbox/cdscompute/cdscompute/cdshandlers/services/handler.py", line 49, in handle_request
result = cached(context.method, proc, context, context.args, context.kwargs)
File "/opt/cdstoolbox/cdscompute/cdscompute/caching.py", line 108, in cached
result = proc(context, *context.args, **context.kwargs)
File "/opt/cdstoolbox/cdscompute/cdscompute/services.py", line 118, in __call__
return p(*args, **kwargs)
File "/opt/cdstoolbox/cdscompute/cdscompute/services.py", line 59, in __call__
return self.proc(context, *args, **kwargs)
File "/home/cds/cdsservices/services/python_service.py", line 32, in execute
raise exceptions.InternalError(logging + traceback, '')
cdsclient.exceptions.InternalError: Traceback (most recent call last):
File "/opt/cdstoolbox/jsonrequest/jsonrequest/requests.py", line 71, in jsonrequestcall
resp = coding.encode(req.callable(*req.args, **req.kwargs), register=encoders, **context)
File "/opt/cdstoolbox/cdstools/cdstools/util.py", line 265, in select
selection = _vertical_selection(selection, vertical)
File "/opt/cdstoolbox/cdstools/cdstools/util.py", line 183, in _vertical_selection
raise ValueError("Invalid vertical declaration '%r'." % vertical)
ValueError: Invalid vertical declaration ''height''.

How can I create subsets of data based on selected index labels along a specified dimension?

The rest of my code can be found here: https://sis-dev.climate.copernicus.eu/toolbox-editor/1474/seasonal-variations-of-humidity 

Many thanks

Huayuan