Fatal error: function call undefined function imagefilter ()

What causes this error?

Fatal error: Call to undefined function imagefilter () in /var/www/polaroid.php on line 5

  1 <?PHP
  2   $img_addr = $_GET['image_address'];
  3   $img = imagecreatefromjpeg($img_addr);
  4
      /* everything works as expected without this line */
  5   imagefilter($img, IMG_FILTER_GRAYSCALE);  
  6
  7   if ($img)
  8   {
        /* I moved the header function here so I can see errors in the browser. 
           If I leave it at the top of the file, the browser expects an image 
           and doesn't print the error messages. */
  9     header('Content-type: image/jpeg');
 10     imagejpeg($img);
 11   }
 12 ?>
 13

This is the URL I'm using:

http: //localhost/polaroid.php? image_address = http: //mattnelsoninfo.files.wordpress.com/2008/04/ceiling_cat1.jpg

NOTE: gd is specified in the output of phpinfo ().

EDIT: I am using PHP Version 5.2.6-2ubuntu4.1

Another EDIT:

phpinfo () gives this in the gd section

gd
GD Support  enabled
GD Version  2.0 or higher
FreeType Support    enabled
FreeType Linkage    with freetype
FreeType Version    2.3.7
T1Lib Support   enabled
GIF Read Support    enabled
GIF Create Support  enabled
JPG Support     enabled
PNG Support     enabled
WBMP Support    enabled 

Thank!