signals return error: is not a function at Object.computation

this is my cart service code:

export class ManageCartItemsService {
cartItems=signal<CartProductModel[]>([]);
item={} as CartProductModel;
}


and this is my code for showing cart items count in other component:


export class CartIconComponent implements OnInit{

cartItemCounter:Signal<number>=computed(()=>this._ManageCartItemsService.cartItems().reduce(
(acc,item)=>acc+item.quantity,0
));

constructor(private _ManageCartItemsService:ManageCartItemsService){}

}

and i get this error:

ERROR TypeError: this._ManageCartItemsService.cartItems is not a function
​

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