when passing a blank extra-var: "Error: failed to parse some of the extra variables."

[erics302]

In ansible-playbook, I can pass a blank variable via cli. I am attempting the same when launching a job_template with tower-cli but get an error.

ansible-tower-cli==3.3.6

$ tower-cli job launch -J my_template_name --extra-vars="foo=''"
Error: failed to parse some of the extra variables.
variables:
foo=''