Angular 2 表单验证，minLength 验证器不起作用

我有以下 Angular 2 形式：

<register>
    <form [ngFormModel] = "registrationForm">
        <div class = "form-group">
            <label class = "control-label" for="email">Email</label>
            <input class = "form-control" type="email" id="email" ngControl="email" #email="ngForm">
        </div>
        <div *ngIf = "email.touched && email.errors">
            <div *ngIf = "!email.errors.required && email.errors.underscoreNotFound" class = "alert alert-danger">
                <span>Underscore is required</span> 
            </div>
            <div *ngIf = "email.errors.required" class = "alert alert-danger">
                <span>Email is required</span>
            </div>
        </div>
        <div class = "form-group">
            <label class = "control-label" for="password">Password</label>
            <input class = "form-control" type="password" id="password" ngControl="password" #password="ngForm">
        </div>
        <div *ngIf = "password.touched && password.errors">
            <div *ngIf = "password.errors.minLength && !password.errors.required" class = "alert alert-danger">
                <span>Password should contain 6 characters</span>
            </div>  
            <div *ngIf = "password.errors.required" class = "alert alert-danger">
                <span>Password is required</span>
            </div>          
        </div>
    </form>
</register>

这是我实现验证器的组件：

import {Component} from '@angular/core';
import {Control, ControlGroup, FormBuilder, Validators} from '@angular/common';
import {CustomValidator} from './CustomValidator';

@Component({
    selector: 'register',
    templateUrl: './app/authentication/register_validation/register.html',
})

export class RegisterComponent{
    registrationForm: ControlGroup;

    constructor(formBuilder:FormBuilder)
    {
        this.registrationForm = formBuilder.group({
            email: ['',Validators.compose([Validators.required, CustomValidator.underscore])], 
            password: ['',Validators.compose([Validators.required,Validators.minLength(6)])]
        });
    }

}

在这种形式下，email字段对于两个验证器都工作正常，即当我不输入任何内容时，它给出"Email is required"消息，当我开始输入内容时，它给出"Underscore is required"消息以及当我输入时"_"所有错误消息都会消失。然而，当我尝试应用这样的 2 个验证器时password场，它不起作用。当我不输入密码时，它会显示以下消息"Password is required"。但是当我输入少于 6 个字符的内容时，minLength消息根本不出现。这段代码有什么问题？

The 错误关键是minlength https://github.com/angular/angular/blob/master/packages/forms/src/validators.ts#L123并不是minLength:

<div *ngIf = "password.hasError('minlength') && !password.hasError('required')" class = "alert alert-danger">
  <span>Password should contain 6 characters</span>
</div>