Wordpress WP_Query() Not working properly

Question

I would like to know why we get an error when we implement this code in our sidebar.php:

$categories = get_categories(); 
foreach($categories as $category) 
{ 
  printf('<h2>%s</h2><ul>', $category->cat_name);
  $posts = new WP_Query('cat='.$category->cat_ID);
  while($posts->have_posts())
  {     
    $posts->the_post();
    echo '<li>', the_title(), '</li>'; 
  }   
  print '</ul>';  
}

The error we are getting is:

Fatal error: Cannot use object of type WP_Query as array in C:\xampp\htdocs\wordpress\wp-includes\query.php on line 2374

Answer 1

sometimes, probably if you have two queries in one page, you need to reset the query data, try to insert the code below after while loop:

wp_reset_postdata();

Answer 2

I was having a similar error when using a WP_Query as given at http://wordpress.stackexchange.com/a/42334/16898.

I replaced the variable name $posts with an arbitrary name $mposts and the error was gone.