[Lazarus] Newbie, porting class to FPC but get procedure assignment error

[Bo Berglund]

I am trying to make a test program in Lazarus to evaluate if I can
port a dongle component in Delphi to a normal class in Lazarus.
So far I had no real problems but now I don't understand what I am
doing wrong. Relevant code below:

TSentinel = class
  private
    FTimer: TTimer;
    ...
    procedure OnCheckTimer(Sender: TObject);
    ...
  public
    { Public declarations }
    constructor Create;
    ...
end;

constructor TSentinel.Create;
begin
  FTimer := TTimer.Create(Application);
  FTimer.Enabled := false;
  FTimer.Interval := 0;
  FTimer.OnTimer := OnCheckTimer(Self); //<= Error here
  ...
end;

The timer is used to automatically check the presence of the dongle so
I have to assign the OnTimer procedure to the newly created TTimer
object. This works fine in Delphi...

On compilation I get an error message as follows.

In the Delphi code there was no argument on the assignment of the
OnTimer event procedure, in Lazarus it shows this error:
sentinel.pas(647,21) Error: Wrong number of parameters specified for
call to "OnCheckTimer"

Using argument Self or NIL (like I often do in Delphi):
sentinel.pas(647,39) Error: Incompatible type for arg no. 1: Got
"untyped", expected "<procedure variable type of procedure(TObject) of
object;Register>"

Why is this difference Delphi/FPC appearing and what can I do?

-- 
Bo Berglund
Developer in Sweden