Error while changing identity for application pool

I try change identity for application pool on Windows Azure. My project uses this application pool when works on Windows Azure. By default application pool uses NetworkService identity, but I must use another identity. I try change it in OnStart() event of WebRole by this way:

using (ServerManager serverManager = new ServerManager())
{                       
    string appPoolName =
    serverManager.Sites[RoleEnvironment.CurrentRoleInstance.Id + "_Web"]
    .Applications.First().ApplicationPoolName;

    var appPool = serverManager.ApplicationPools[appPoolName];

    appPool.ProcessModel.UserName = Environment.MachineName + "\\UserName";

    appPool.ProcessModel.IdentityType = ProcessModelIdentityType.SpecificUser;

    appPool.ProcessModel.Password = "UserPassword";

    serverManager.CommitChanges();
}

But I get exception with next message:

    System.Runtime.InteropServices.COMException (0x80090016): 
       Keyset does not exist (Exception from HRESULT: 0x80090016)
   at Microsoft.Web.Administration.Interop.AppHostWritableAdminManager.CommitChanges()
   at Microsoft.Web.Administration.Configuration.CommitChanges()
   at Microsoft.Web.Administration.ConfigurationManager.CommitChanges()
   at Microsoft.Web.Administration.ServerManager.CommitChanges()
   at Project.Web.WebRole.OnStart() in E:\Projects\...\Web\WebRole.cs:line 57

If I change identity in IIS manager I don't get any error. What is wrong with my code and why do I get this error?

Updates to the applicationHost.config require administrative privileges. When you run locally, you are an administrator. In the cloud, your RoleEntryPoint runs as a normal user unless you elevate the role. Have you done so?

Check to see if you have <Runtime executionContext="elevated"/> specified inside your role declaration in ServiceDefinition.csdef.

Edit: Wade also showed how to do this using a slightly different method (check the comments). Try this as well