diff --git a/packages/block-serialization-default-parser/parser.php b/packages/block-serialization-default-parser/parser.php
index 2211cf0c192260..edda34b571568d 100644
--- a/packages/block-serialization-default-parser/parser.php
+++ b/packages/block-serialization-default-parser/parser.php
@@ -457,7 +457,7 @@ function add_freeform( $length = null ) {
 	 */
 	function add_inner_block( WP_Block_Parser_Block $block, $token_start, $token_length, $last_offset = null ) {
 		$parent = $this->stack[ count( $this->stack ) - 1 ];
-		$parent->block->innerBlocks[] = $block;
+		$parent->block->innerBlocks[] = (array) $block;
 		$html = substr( $this->document, $parent->prev_offset, $token_start - $parent->prev_offset );
 
 		if ( ! empty( $html ) ) {