我不断收到以下控制台错误：

Error trying to diff 'Paul'. Only arrays and iterables are allowed

HTML：

<fieldset class="one-quarter sm-padding">
    <label>Occupation</label>
    <select name="occupations"
            [(ngModel)]="occupations" 
            (ngModelChange)="showValuePromptText('Occupation', $event)">
        <option *ngFor="let occupation of occupations" 
                [value]="occupation">
            {{occupation}}
        </option>
    </select>
</fieldset>

TS：

import { Component } from '@angular/core';
import { PromptService } from '../../services/prompt.service';
import { IPromptText } from "../../models/IPromptText";

@Component({
    selector: 'fact-find',
    templateUrl: './factfind.component.html',
    styleUrls: ['./factfind.component.css'],
    providers: [PromptService]
})

export class FactFindComponent {
    promptTexts: IPromptText[];
    currentPromptText : string;

    showDefaultPromptText(fieldName: string) {
        if (fieldName != null) {
            this.promptService.fetchPrompts(fieldName, '').subscribe(
                (data) => this.currentPromptText = data
            );
        }
    }

    showValuePromptText(fieldName: string, fieldValue: string) {
        if (fieldValue != null) {
            this.promptService.fetchPrompts(fieldName, fieldValue).subscribe(
                (data) => this.currentPromptText = data
            );
        }
    }

    occupations: string[] = ["John", "Paul", "George", "Ringo"];
}

如果有人可以告诉我如何修复此问题，我将不胜感激。