Rendering JSON for Multiple Rails Objects

This may be a terribly simple question, but this is what I noticed that worries me.

I'm trying to display JSON from one of my controller's methods, but it gives me an undefined `new 'for nil: NilClass method.

Here is the code causing the problem:

def index
    @users = User.all
    render json: @users
end

      

        
        
        
      

    

I noticed that when I try to display only one object for JSON everything works fine:

def show
    @user = User.find(params[:id])
    render json: @user
end

      

        
        
        
      

    

Or, when I call to_json

on the @users object:

def index
    @users = User.all
    render json: @users.to_json
end

      

        
        
        
      

    

I was under the impression that the call was render json:

implicitly calling to_json

anyway, so why call, which twice solves my problem?