Angular4表单验证问题

[zxp1103]

前端代码如下：

<form *ngIf="desc" (ngSubmit)="f.form.valid && onCreate()" #f="ngForm" novalidate>
  <div class="help-block error box" *ngIf="errors">
    <ul>
      <li *ngFor="let error of errors">{{ error }}</li>
    </ul>
  </div>
  <ng-template ngFor let-field [ngForOf]="desc.fields">
    <div class="form-group" *ngIf="field.isEditableOnCreate" [ngClass]="{ 'warn': f.submitted && input && !input.valid }" [ngSwitch]="field.dataType">
      <label for="{{ field.name }}">{{ field.displayName }}</label>
      <div class="select" *ngIf="referenceFields && referenceFields[field.name]; else elseBlock">
        <select name="{{ field.name }}" [(ngModel)]="model[field.name]" #input="ngModel">
          <option *ngFor="let opt of referenceFields[field.name]" [value]="opt.id">{{ opt.displayName || opt.name || opt.internalReferenceId }}</option>
        </select>
      </div>
      <ng-template #elseBlock>
        <input *ngSwitchCase="EntityFieldDataType.boolean" type="checkbox" class="form-control" name="{{ field.name }}" [(ngModel)]="model[field.name]"
          #input="ngModel" />
        <textarea *ngSwitchCase="EntityFieldDataType.text" type="text" class="form-control" name="{{ field.name }}" [(ngModel)]="model[field.name]"
          #input="ngModel"></textarea>
        <input *ngSwitchCase="EntityFieldDataType.dateTime" type="date" class="form-control" name="{{ field.name }}" [(ngModel)]="model[field.name]"
          #input="ngModel" />
        <input *ngSwitchDefault type="text" class="form-control" name="{{ field.name }}" [(ngModel)]="model[field.name]" #input="ngModel"
          [required]="field.isRequired" />
      </ng-template>
      <div *ngIf="f.submitted && !input.valid" class="help-block">请填写{{ field.displayName }}</div>
    </div>
  </ng-template>
  <ng-content></ng-content>
  <div class="form-group">
    <button type="submit" class="btn btn-primary">{{ createText }}</button>
  </div>
</form>

总是提示：Cannot read property 'valid' of undefined。
请问问题出在哪里？