Timer won't stop

I have a simple Timer which I want to stop once certain validations are complete via HTTP, but it won't stop.

To initiate:
appStartTimer = new Timer.Timer();
appStartTimer.start(method(:appStartCallback), 1000, true);

On completion of the validation process (a successful HTTP response):
appStartTimer.stop;
The stop code is being hit (I have a Sys.println before), but it fails to stop (I have a Sys.println in appStartCallback)

Running SDK 2.3.3 on a VA-HR fw 4.0