Getting this error, someone please help me out

Module: Apex Triggers -> Bulk Apex Triggers

I'm trying to complete the challenge from the above module. I'm getting the following error when I check the challenge. 

Error: We created an opportunity and expected it to have an associated task, but it didn’t. Make sure your Apex trigger inserts the task into the database.

As per the instruction, I've created the bulk apex trigger for Opportunity Object.

trigger ClosedOpportunityTrigger on Opportunity (after insert, after update) {
List<Task> tasks = new List<Task>();
for(Opportunity opp: Trigger.New){
if(opp.StageName == 'Closed Won'){
Task task = new Task(Subject='Follow Up Test Task', WhatId=opp.Id);
tasks.add(task);
}
}
if (tasks.size() > 0) {
insert tasks;
}
}

I've manually created an Opportunity with Stage Closed Win and I can see the follow-up task is getting created for the same.