Why can't my query object use _id properly?

I'm trying to write a Node.js web API using Express and MongoDB. I'm dynamically parsing the req.query object (Express's querystring storage/parser) from a GET request and updating an object called "query" with the the querystring pairs so I can find documents in the database collection that match whatever the querystring parameters are. For example, a localhost:3000//users?name=John&last=Smith will modify the query object so that we find only records that have a name field as John and a last field as Smith. We could also send none of those parameters, only one, or more. The problem is when I try to send a querystring like this:

localhost:3000//users?_id=1

This returns nothing. When I use the terminal (windows cmd) to directly access the database and run this query (db.Users.find({_id : 1})) it works fine and returns as expected. Further, console.log(query._id) returns 1, so I know that the object is storing the parameter as I expect.

var query = {};
//Iterate through query string parameters as a Get request
//Add them to the query object, (add them as object properties)
for(var propName in req.query) {
    query[propName] = req.query[propName];
}
db.collection("Users", function(err, collection) {
        if(!err) {
            collection.find(query, {limit: 100},function(err, cursor) {
                if(!err) {
                    cursor.toArray(function(err, arr) {
                        if(!err) {
                            console.log(arr);
                            res.send(arr);
                        } else {
                            console.log("Failed to turn cursor to array: "+err);
                        }
                    });
                } else {
                    console.log("Find failed: "+err);
                }
            });
        } else {
            console.log("Failed opening collection: "+err);
        }
    });