Cannot add property 0, object is not extensible, when selecting new items in multiple groupby mode

[reinoboonstra]

Getting this js error 'Cannot add property 0, object is not extensible' when selecting an item in my list.
Context:
This is my HTML: <angular2-multiselect [data]="countriesRegionsData" formControlName="countriesRegions" [settings]="settings">

These are my settings:
public settings = {
singleSelection: false,
text: 'Select Fields',
selectAllText: 'Select All',
unSelectAllText: 'UnSelect All',
searchPlaceholderText: 'Search Fields',
enableSearchFilter: true,
badgeShowLimit: 5,
primaryKey: 'countryCode',
labelKey: 'countryName',
groupBy: 'regionName'
};
This is the object I use:
export interface ICountryRegion {
regionId: string;
regionName: string;
countryCode: string;
countryName: string;
}
I use version 5.0.4
It goes wrong on line 1649 of angular2-multiselect-dropdown.js: this.selectedItems.push(item);
This could probably be replaced by [...this.selectedItems, item]; i.e. making a new copy of the array.