An exception occuring while creating summary task and task information in Microsoft Project via Add-In using VSTO

Question

In my VSTO(Visual Studio Tools for Office) Add-in for Microsoft Project, I'm getting this error:

System.Runtime.InteropServices.COMException: 'The argument value is not valid.'

I am trying to Add the Summary Tasks, and Tasks Information. The exception occurs in this block of code.

 countryNameSummaryTask.OutlineChildren.Add(regionNameSummaryTask);

Here is my code:

private void button1_Click(object sender, RibbonControlEventArgs e)
{
Project project = Globals.ThisAddIn.Application.ActiveProject;

// Create Country Name Summary task
string countryName="US";
Task countryNameSummaryTask = project.Tasks.Add(countryName);
countryNameSummaryTask.OutlineLevel = 1;

// Create State/Region Name Summary task
string regionName="TN";
Task regionNameSummaryTask = project.Tasks.Add(regionName);
regionNameSummaryTask.OutlineLevel = 2;
countryNameSummaryTask.OutlineChildren.Add (regionNameSummaryTask);

// Adding list of tasks
var phases = new List<Phase>(){
new Phases() { Name="P11", Start=DateTime.Now, Finish=DateTime.Now.AddDays(2)},
new Phases() { Name = "P22",Start=DateTime.Now, Finish=DateTime.Now.AddDays(2) },
new Phases() { Name = "P33", Start=DateTime.Now, Finish=DateTime.Now.AddDays(2) },
new Phases() { Name = "P44", Start=DateTime.Now, Finish=DateTime.Now.AddDays(2) }, 
}

foreach (var phase in phases)
{
 Task newPhaseTask = project.Tasks.Add(phase.Name);
 newPhaseTask.OutlineLevel = 3;
 regionNameSummaryTask.OutlineChildren.Add(newPhaseTask);
}

}