How to use WebArgumentResolver for assigning params with level of indirection

I have a question about WebArgumentResolver. I added property emailSetting to user domain class,

EmailSetting emailSetting = new EmailSetting()

class for it:

class EmailSetting {
    boolean newsletter = true
    boolean mayorships = true
    boolean comments = true
}

now I want to edit/update these settings and assign params to it in UserService:

user.emailSetting = new EmailSetting(params.emailSetting)

the params come from a form in edit.gsp and are named

emailSetting.comments
emailSetting.mayorships
emailSetting.newsletter

what I expect the WebArgumentResolver to do is assign the values. But maybe as it has one level of indirection. logging the params shows:

 _emailSetting.mayorships:, emailSetting:[_comments:, _newsletter:, newsletter:on, _mayorships:, comments:on], emailSetting.newsletter:on, _emailSetting.comments:, email

error message is:No such property: _comments for class: com.shopotainment.util.EmailSetting

Whats wrong or could be changed to fix this?