'Find documents with $gte or $lte which has numbers as string values in arrays

I have a MongoDB collection that contains documents as follows:

{
 "_id": ObjectId(.....),
 "name": "John",
 "contacts": [
    {
      "name": "henry",
      "age": "22"
    },
    {
      "name": "merry",
      "age": "12"
    }
 ]
}

I want to find documents which have contacts.age is less than 20. I tried the following mongo query but had no luck. Anyone can help me out?

document.Users.find({'$expr':{'$lte': [{'$toInt':'$contacts.age'}, '20'] }})

Above query gives following error:

Query failed with error code 241 and error message 'Executor error during find command :: 
caused by :: 
Unsupported conversion from array to int in $convert with no onError value' on server
mongodbmongodb-queryaggregation-framework


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