<form novalidate>
  <h4>Inside a form</h4>

  <md-input-container>
    <input mdInput name="example" placeholder="example"
      [(ngModel)]="errorMessageExample3" required>
    <md-error>This field is required</md-error>
  </md-input-container>

  <button color="primary" md-raised-button>Submit</button>
</form>

Expected result

不幸的是,当我尝试做同样的事情时,我得到：

Unhandled Promise rejection: Template parse errors:
'md-error' is not a known element:
1. If 'md-error' is an Angular component,then verify that it is part 
   of this module.
2. If 'md-error' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' 
   to the '@NgModule.schemas' of this component to suppress this message.

我该怎么做才能使用< md-error>？

@NgModule({
    imports: [
        ...
        MaterialModule
    ],...
})