I tried the checlist field with the following code:

piklist('field', array(
    'type' => 'checkbox',
    'field' => 'my_checkbox',
    'label' => 'Checkbox',
    'value' => 'third', // set default value
    'choices' => array(
      'first' => 'First Choice',
      'second' => 'Second Choice',
      'third' => 'Third Choice'
    )
  ));

When I see the code from the browser, I found the following result:

<li>    
      <label class="piklist-field-list-item">  
        <input 
          type="checkbox"
          id="my_checkbox_0" 
          name="my_checkbox[]"
          value="first"
                    class="my_checkbox piklist-field-type-checkbox piklist-field-element"         />         
          <input 
            type="hidden"
            id="my_checkbox_0" 
            name="my_checkbox[]"
            value=""
          />
      
                
        <span class="piklist-list-item-label">
          First Choice        </span>
    
      </label>
  
    </li>

There is a hidden field come out from the code. What should I do to correct this?