conversions to dates don't accept constant arguments

var x = db.runCommand({aggregate: "foo", pipeline: [

  {$project: {

//     doesn't work

//      date: {$isoDate: {year: 2012, month: 2, dayOfMonth: 4} } ,

/* // this worked - copy-pasted from testall.js

      testDate: {$isoDate:{

          year: "$year", month: "$month", dayOfMonth: "$dayOfMonth",

          hour: "$hour", minute: "$minutes", second: "$seconds"}}

*/

// this didn't

      testDate: {$isoDate:{

          year: 2012, month: "$month", dayOfMonth: "$dayOfMonth",

          hour: "$hour", minute: "$minutes", second: "$seconds"}}

  }}

]});

{

        "errmsg" : "exception: \"year\" numeric inclusion or exclusion must be 1

 or 0 (or boolean)",

        "code" : 15991,

        "ok" : 0

}