This script causes an error:

db = db.getSiblingDB('foo');

var r = db.runCommand({ "aggregate" : "blah", "pipeline" : [
{ "$project" : {
"key" : 1,
"v" : 1
}},
{ "$group" : {
"_id" :

,
"min_v" :

}}
]});

> r
{
"errmsg" : "exception: disallowed field type 16 in object expression (at
\"key\")",
"code" : 15992,
"ok" : 0
}

I tried using NumberInt for the ones in the $project, and they were fine. Just the one in the $group has the problem.