I am trying to use a search from

1. authors,
2. listings(i.e custom post type)
3. products(i.e custom post type)

I am able to search for custom post types but unable to do so for authors.

I am using the following search form.

<form role="search" method="get" id="searchform" action="<?php echo home_url( '/' ); ?>">                  
   <div class="input-box">
       <input name="s" id="s" type="text" class="form-control" placeholder="What you are looking for...">
    </div>

    <div class="selection-box">
         <select name="post_type" id="basic" class="selectpicker show-tick form-control">
             <option value="job_listing">Listing</option>
             <option value="product">Shop</option>
             <option value="author">Author</option>
             <option value="">All</option>                        
         </select>                      
     </div>

     <div class="search-btn">
          <input type="submit" class="form-control" value="search">
     </div>
</form>