On project create, membership_ids is not valid

[devanp92]

Here is the error:
[Error]
status: 400,
kind: 'error',
code: 'invalid_parameter',
error: 'One or more request parameters was missing or invalid.',
requirement: '',
generalProblem: 'this endpoint cannot accept the parameter: membership_ids',
possibleFix: '',
validationErrors: '',
message: '' }

In the docs you allow passing membershipIds on project create.

The project I am creating is:

var proj = {

name: "Test",
membershipIds: [9999,9856]

}

Where membershipIds are actual ids.