Unlock Account on Active Directory

I'm experiencing some problems when unlocking a locked account on AD.

I'll keep it very simple. I'm assuming that the account is currently locked.

UserPrincipal oUserPrincipal = ADMethods.GetUser("test.user");
oUserPrincipal.UnlockAccount();
oUserPrincipal.Save();

oUserPrincipal is returned using an Admin account on the AD though a PrincipalContext using

Now

oUserPrincipal.UnlockAccount(); is generating an exception System.UnauthorizedAccessException

When checking the logs on the "Test AD server", it's returning a failed negotiation stating that "test.user" is locked out. Since I'm returning a user principal using an admin account, why is unlockAccount method generating an error?

Would appreciate your help. attached please find the entire class

here's the exception:

System.UnauthorizedAccessException: Access is denied.

at System.DirectoryServices.Interop.UnsafeNativeMethods.IAds.SetInfo()

at System.DirectoryServices.DirectoryEntry.CommitChanges()

at System.DirectoryServices.AccountManagement.SDSUtils.WriteAttribute(String dePath, String attribute, Int32 value, NetCred credentials, AuthenticationTypes authTypes)

at System.DirectoryServices.AccountManagement.ADStoreCtx.WriteAttribute(Principal p, String attribute, Int32 value)

at System.DirectoryServices.AccountManagement.ADStoreCtx.UnlockAccount(AuthenticablePrincipal p)

at System.DirectoryServices.AccountManagement.AccountInfo.UnlockAccount()

at UnlockApp.Form1.UnlockUserAccount(String sUserName) in C:\Users\christian.debono\Documents\Visual Studio 2010\Projects\UnlockApp\UnlockApp\Form1.cs:line 78

at UnlockApp.Form1.UnlockButton_Click(Object sender, EventArgs e) in C:\Users\christian.debono\Documents\Visual Studio 2010\Projects\UnlockApp\UnlockApp\Form1.cs:line 45

at System.Windows.Forms.Control.OnClick(EventArgs e)

at System.Windows.Forms.Button.OnClick(EventArgs e)

at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)

at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

at System.Windows.Forms.Control.WndProc(Message& m)

at System.Windows.Forms.ButtonBase.WndProc(Message& m)

at System.Windows.Forms.Button.WndProc(Message& m)

at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)

at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)