node.js mongodb client :Cannot call method 'collection' of null

this is my data.config file

DbProvider.prototype.doOperation = function(collection, operation, params, callback)
 {
    this.validateParams(params, callback);

var operationCallback = function(err, result)
{

    callback(err, result);
};
this.db.collection(collection, function(err, collection) {
    if(operation==='find') {
        collection.find().toArray(operationCallback);
    } else {
        collection[operation](params, operationCallback);
    }
});
}

DbProvider.prototype.findUser = function(params, callback)
 {

   this.doOperation('users', 'findOne', params, callback);

 };

i have tried to find id exit or not by this way

dbProvider.findUser({_id:"514fdb464f2c06a9d6003afb"},function(err,result)

{

console.log("welcome");

});

but i got this error Cannot call method 'collection' of null how to resolve this...