Run with another uid seems not working

[pioz]

I need to check a program that has a systemd unit under user namespace and this is my monit conf:

check program foo with path "/usr/bin/systemctl --user -q is-active foo" as uid pioz and gid pioz
  start program = "/usr/bin/systemctl --user start foo" as uid pioz and gid pioz
  stop program = "/usr/bin/systemctl --user stop foo" as uid pioz and gid pioz
  if status != 0 then restart
  if 5 restarts within 5 cycles then timeout

This conf does not work because I get the error Failed to connect to bus: No such file or directory. I get the same error if I run the follow command sudo /usr/bin/systemctl --user -q is-active foo, so seems that the directive as uid pioz and gid pioz does not work. If I change the config like this, all works well.

check program foo with path "/bin/su -c '/usr/bin/systemctl --user -q is-active foo' pioz"
  start program = "/bin/su -c '/usr/bin/systemctl --user start foo' pioz"
  stop program = "/bin/su -c '/usr/bin/systemctl --user stop foo' pioz"
  if status != 0 then restart
  if 5 restarts within 5 cycles then timeout

Any idea why as uid does not work?