__pillar___ empty in custom module - salt-stack

Question

I write a simple custom module, and would like to access the pillar data.
I test it like this (file salt/_modules/foo.py):
def bar():
assert 0, __pillar__
sls file salt/tmp/test_pillar_in_custom_module.sls:
{{ salt['foo.bar']() }}
And I execute it like this:
salt-ssh --log-leve=all foohost state.sls tmp.test_pillar_in_custom_module pillar='{systems: [foosys]}' 2> tmp/o
Relevant part of tmp/o:
Traceback (most recent call last):
File "/var/tmp/.root_dcdf8c_salt/salt-call", line 27, in <module>
salt_call()
File "/var/tmp/.root_dcdf8c_salt/pyall/salt/scripts.py", line 431, in salt_call
client.run()
File "/var/tmp/.root_dcdf8c_salt/pyall/salt/cli/call.py", line 57, in run
caller.run()
File "/var/tmp/.root_dcdf8c_salt/pyall/salt/cli/caller.py", line 138, in run
ret = self.call()
File "/var/tmp/.root_dcdf8c_salt/pyall/salt/cli/caller.py", line 237, in call
ret['return'] = self.minion.executors[fname](self.opts, data, func, args, kwargs)
File "/var/tmp/.root_dcdf8c_salt/pyall/salt/executors/direct_call.py", line 12, in execute
return func(*args, **kwargs)
File "/var/tmp/.root_dcdf8c_salt/running_data/var/cache/salt/minion/extmods/modules/foo.py", line 2, in bar
assert 0, __pillar__
AssertionError: {}
But the dict __pillar__ is empty.
I read the docs: https://docs.saltstack.com/en/latest/topics/development/modules/developing.html#pillar
The __pillar__ dictionary contains the pillar for the respective
minion.
While __pillar__ is defined for every module, it's only filled in for
some.
In my custom module it is empty.
Is there a way to access the pillar in a custom module?