Dimensional alias: "--config option cannot be shortened" error

I defined the following alias in my mercury configuration.

[alias]
ci_s = ci --config phases.new-commit=secret

When I ran the command ci_s, I got an error:

abort: the --config option cannot be shortened!

Not sure what causes the error. My guess is the "=" sign between the new-fixation and the secret. Is there a way to make this alias work? Thank.