[SBE] Undefined slot error for $group with $cond / $switch / $ifNull expressions

db.b.explain().aggregate([

  {$match: { master: 1} },

  {$group: {"_id":"$status", "counter":{$sum:1}}},

  {$group: {

    "_id": null,

    "pending":{$sum: {$cond:[{$lte:["$_id",99]},"$count",0]}},

    "running":{$sum: {$cond:[{$gte:["$_id",100]},"$count",0]}}

  }}

]);