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#56941 closed defect (bug) (worksforme)

Fatal error produced by block using closure as render_callback

Reported by: andrewleap's profile andrewleap Owned by:
Milestone: Priority: normal
Severity: normal Version:
Component: Themes Keywords:
Focuses: Cc:


The following code causes site to blow up on front and in post editor

                                     // attempted to serialize closure      
		'render_callback' => function( $attributes, $content, $block ){}

Fatal error: Uncaught Exception: Serialization of 'Closure' is not allowed in /app/wp-includes/block-supports/elements.php...

Change History (5)

#1 follow-up: @wildworks
19 months ago

I could not reproduce it in either WordPress 6.0 or 6.1-RC6. Did it work correctly in WordPress 6.0?

Here is my test procedure:

  • Run npx @wordpress/create-block test-block directly under the plugin directory
  • Update test-block.php as follows:
function create_block_test_block_block_init() {
		__DIR__ . '/build',
			'render_callback' => function( $attributes, $content, $block ) {
				return 'rendered';
add_action( 'init', 'create_block_test_block_block_init' );
  • Insert the block into the editor and save it
  • Confirm that no errors occur in both the editor and the front end

#2 @audrasjb
19 months ago

@andrewleap thanks for the ticket!

Could you please provide more info about the WordPress version you used when you produced this issue? Thanks :)

#3 in reply to: ↑ 1 @andrewleap
19 months ago

Apologies, This was my mistake. I was panicking as the new version is being released imminently .

The culprit was actually this in my theme:

 * Add a reference to block parent in render function.
 * @param array $parsed_block Block data.
function xd_append_parent_block_data( $parsed_block ) {
	$block = new WP_Block( $parsed_block );
	foreach ( $parsed_block['innerBlocks'] as &$inner_block ) {
		$inner_block['parent'] = array(
			'name'  => $block->name,
			'props' => $block->attributes,
			'block' => $block,
	return $parsed_block;

add_filter( 'render_block_data', 'xd_append_parent_block_data' );

We add a reference to the parent block in the block data as this allows us to contextually change the output in the render function. That parent block will also have a closure for its render callback function. Of Course, the block serialization that was recently added will not play nice when trying to serialize a callback.

function wp_get_elements_class_name( $block ) {
        return 'wp-elements-' . md5( serialize( $block ) );

We can fix this with update to our plugin. Thanks for taking the time to look at this.

#4 @audrasjb
19 months ago

  • Keywords reporter-feedback added
  • Milestone changed from Awaiting Review to 6.1

Moving to milestone 6.1 for better visibility, while we're waiting for reporter feedback.

(if the issue is confirmed, this ticket will probably move to 6.1.1 though)

Last edited 19 months ago by audrasjb (previous) (diff)

#5 @audrasjb
19 months ago

  • Keywords reporter-feedback removed
  • Milestone 6.1 deleted
  • Resolution set to worksforme
  • Severity changed from blocker to normal
  • Status changed from assigned to closed
  • Version 6.1 deleted

Closing as per comment 3 :)

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