mongodb $exists not returning anything

My MongoCollection is:

{
    _id: ObjectId("560150b01f4da0390f10700a"),
    name: "Zouk Bar & Grill ",
    costing: {
        hallcost: 100,
        vegcost: 900,
        nonvegcost: 900
    },
    occasionwisecosting: {
        WeddingCost: 900,
        OfficePartyCost: 900,
        AnniversaryCost: 900,
        BirthdayPartyCost: 900,
        GettogetherCost: 900
    }
}

I am trying to retrieve the rows where we have GettogetherCost with the following query

db.property_master.find({'occasionwisecosting.WeddingCost' : {$exists : true }});

However I am getting an empty resultset:

> db.property_master.find({'occasionwisecosting.WeddingCost' : {$exists : true }});
>

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