Ember set value in object of array from Service

I'm trying to set the attribute 'show' of an object, that comes from an array which is served by a service.

the service (not the problem):

// services/events.js
export default Ember.Service.extend({
types: [
    {"code": 0, "label": 'Performance', "show": true, "css": 'warning'},
    {"code": 1, "label": 'Preview', "show": true, "css": 'danger'},
    ....
],
});

in my controller I have an action that should update the attribute 'show':

// controller
const { service } = Ember.inject;

export default Ember.Controller.extend({
    eventsService: service('events'),
    ....
    actions: {
        setEventFilter: function(code) {
            ....
            let obj = this.get('eventsService.types').objectAt(code);
            // I get a proper Object here, that also has a set-function:
            console.log(obj);
            // this fails with 'Uncaught TypeError: obj.set is not a function':
            obj.set('show', false);
        }
    }
});

I've read the answers here Ember: set array value for specific key , tried all the possibilities, nothing worked in my case.

What I don't understand is, that I can see the function 'set' in console, yet I still get that error:

Any suggestions? Anybody got an idea what I'm doing wrong?