I get an error Password can not be blank ruby on rails 5

I am using Rails and angular 2 for my app, in my api rails I have added the

gem 'bcrypt', '~> 3.1.7'

my database table has password_digest: string and my model has_secure_password .

When I sent my form I get an error

  

Password can not be blank

Rails controller :

class UsersController < ApplicationController

    def create 
        @user = User.new(user_params)

        if @user.save 
            session[:user_id] = @user.id 
            render json: @user, status: :created, location: @user
        else 
            render json: @user.errors, status: :unprocessable_entity
        end 
    end

    private
    def user_params
        params.require(:user).permit(:first_name, :last_name, :email, :password)
    end
end

Angular2 :

ngOnInit() {
    this.Form = this.fb.group({
        first_name: this.fb.control(null, Validators.required),
        last_name: this.fb.control(null, Validators.required),
        email: this.fb.control(null, Validators.required),
        password: this.fb.control(null, Validators.required)
    });
}