title: Laravel multi auth - Authentication user provider [] is not defined
body: <p>It works when it put </p>
  
  <pre><code>'merchant' =&gt; [
          'driver' =&gt; 'session',
          'provider' =&gt; 'users',
      ],
  </code></pre>
  
  <p>But It shows error when I set a different table </p>
  
  <pre><code>'merchant' =&gt; [
          'driver' =&gt; 'session',
          'provider' =&gt; 'merchants',
      ],
  </code></pre>
  
  <p>Error: 
  InvalidArgumentException in CreatesUserProviders.php line 40: Authentication user provider [] is not defined.</p>
  
  <p>What is the problem. Would you please explain me ?
  Thanks.</p>