Type error in Service Routine

[this.wiederkehr]

I randomly get the following error:

Unhandled exception in callback handler
TypeError: 'bytearray' object is not callable

or

Unhandled exception in callback handler
TypeError: 'slice' object is not callable

This happens in a callback which is triggerd by a pin interrupt. Defined like so:

int_pin.callback(trigger=Pin.IRQ_RISING, handler=self.handler)

The callback function is a very simple method of an object:

def handler(self, arg):
    self.todo = True

Actually, there is no such type as slice or bytearray involved in the callback which makes me think its a bug ...

Sometimes the callback works fine for several thousands of times, some times for just a few hundreds.
After that exception the callback seems to be never called again, even tough it should have been called again.

I have a main loop which looks as follows and shall handle the todos:

while True:
    irq = machine.disable_irq()
    if this.todo:
        this.todo = False
        do_this()
    if that.todo:
        that.todo = False
        do_that()
    machine.enable_irq(irq)
    time.sleep_us(1)

Any ideas?

Oh yea, forgot to mention:

(sysname='LoPy', nodename='LoPy', release='1.6.12.b1', version='v1.8.6-593-g8e4ed0fa on 2017-04-12', machine='LoPy with ESP32', lorawan='1.0.0')