Checkbox binding not working in ionic 2

let’s say catDict is a dictionnary of type :
public catDict: { [id: string]: boolean; }

if I bind catDict on a checkbox like here :
<ion-checkbox checked="catDict[p._id]" (change)="updateSelected(p._id)"></ion-checkbox>
also tried
<ion-checkbox [checked]="catDict[p._id]" (change)="updateSelected(p._id)"></ion-checkbox>

I expect the check box to check or uncheck itself reactively based on the value stored in catDict[p._id]

but nothing happens, am I doing something wrong ? if I use ngModel instead of checked my screen freezes

I’m using Ionic2 : “ionic-angular”: “2.0.0-beta.7”