Angular select 标签 Error trying to diff '2'. Only arrays and iterables are allowed

在表单书写的时候，如果把select 标签的绑定成*ngFor 的数组，会报类似
Error trying to diff ‘2’. Only arrays and iterables are allowed的错误。需要把绑定的数组修改为数组中的item
错误示范

   <div class="form-group">
        <label for="partners">Hero partners</label>
        <select class="form-control" id="partners" [(ngModel)]="partners" name="partners">
            <option *ngFor="let partner of partners" [value]="partner.partnerId">{{partner.partnerName}}</option>
        </select>
    </div>

修改为

   <div class="form-group">
        <label for="selectedPartner">Hero partners</label>
        <select class="form-control" id="selectedPartner" [(ngModel)]="selectedPartner" name="selectedPartner">
            <option *ngFor="let partner of partners" [value]="partner.partnerId">{{partner.partnerName}}</option>
        </select>
    </div>