PHP urlencode() breaks filter function

I’m trying to get the following code to work in my plugin:

function ell_gmap_add_media_button( $context ) {
	global $post;
	$post_id = $post->ID;
	//global $wpdb;

	$ell_gmap_media_button = '%s' . '<a href="media-upload.php?';

	// Get saved Geotag data from post if present
	$field = 'ell_gmap_geotag_data';
	$saved_geotag_data = get_post_meta($post_id, 'ell_gmap_geotag_data', true);
	if( $saved_geotag_data ) {
		$saved_values = explode( ',', $saved_geotag_data );
		$lat          = str_replace( 'lat=', '', $saved_values[0] );
		$lng          = str_replace( 'lng=', '', $saved_values[1] );
		$title        = str_replace( 'title=', '', $saved_values[2] );
		$title = urlencode( $title ); // <-- Breaks script
		$description  = str_replace( 'description=', '', $saved_values[3] );
		$ell_gmap_media_button .= "lat=$lat&lng=$lng&title=$title&description=$description&";

	}

	$ell_gmap_media_button_image = get_plugin_dir() . '/img/mediabutton.png';
	$ell_gmap_media_button .= "post_id=$post_id&tab=ell_insert_gmap_tab&TB_iframe=true\" class=\"thickbox\" title=\"Geotag Post\"><img src=\"$ell_gmap_media_button_image\" /></a>";

	return sprintf($context, $ell_gmap_media_button);
}
add_filter( 'media_buttons_context', 'ell_gmap_add_media_button' );

However, when I add the urlencode() function, this part of the script breaks (ie the extra Media Button isn’t added, and in fact, all the Media Buttons disappear. Removing the urlencode() function restores the intended functionality. The title and description vars from the post custom field may contain spaces and special characters, and since I need to pass them in the URL string, I’d like to make sure they’re properly URL-encoded.

What am I doing wrong?

a|x