MongoDB – The argument to $size must be an Array, but was of type: EOO / missing

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You can use the $ifNull operator here. It seems the field is either not an array or not present by the given error:

{ "$project": {
    "people": 1,
    "Count": { 
        "$size": { "$ifNull": [ "$myFieldArray", [] ] }
    }
}}

Also you might want to check for the $type in your $match in case these do exist but are not an array.

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