diff --git a/src/wp-includes/class-wp-block-parser.php b/src/wp-includes/class-wp-block-parser.php
index 06342fc5acb64..13c5ac2029518 100644
--- a/src/wp-includes/class-wp-block-parser.php
+++ b/src/wp-includes/class-wp-block-parser.php
@@ -66,11 +66,31 @@ class WP_Block_Parser {
 	 * return an error on invalid inputs.
 	 *
 	 * @since 5.0.0
+	 * @since 7.2.0 Triggers a notice when the document is not a string, and returns
+	 *              an empty array unless the document is a stringable object.
 	 *
 	 * @param string $document Input document being parsed.
 	 * @return array[]
 	 */
 	public function parse( $document ) {
+		if ( is_object( $document ) && method_exists( $document, '__toString' ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Convert the document to a string before calling parse().' ),
+				'7.2.0'
+			);
+			$document = (string) $document;
+		}
+
+		if ( ! is_string( $document ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'The document must be a string.' ),
+				'7.2.0'
+			);
+			return array();
+		}
+
 		$this->document = $document;
 		$this->offset   = 0;
 		$this->output   = array();
diff --git a/tests/phpunit/tests/blocks/wpBlockParser.php b/tests/phpunit/tests/blocks/wpBlockParser.php
index 07fdb556efcc4..37da69265b228 100644
--- a/tests/phpunit/tests/blocks/wpBlockParser.php
+++ b/tests/phpunit/tests/blocks/wpBlockParser.php
@@ -42,6 +42,68 @@ public function test_parses_basic_empty_block() {
 		);
 	}
 
+	/**
+	 * Ensures that a document that is not a string returns an empty array and triggers a notice.
+	 *
+	 * @ticket 66138
+	 *
+	 * @covers ::parse
+	 *
+	 * @dataProvider data_non_string_documents
+	 *
+	 * @expectedIncorrectUsage WP_Block_Parser::parse
+	 *
+	 * @param mixed $document Document that is not a string.
+	 */
+	public function test_returns_empty_array_for_non_string_document( $document ) {
+		$p = new WP_Block_Parser();
+
+		$this->assertSame( array(), $p->parse( $document ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public static function data_non_string_documents() {
+		return array(
+			'null'    => array( null ),
+			'false'   => array( false ),
+			'integer' => array( 123 ),
+			'array'   => array( array( 'content' ) ),
+			'object'  => array( new stdClass() ),
+		);
+	}
+
+	/**
+	 * Ensures that a stringable object is parsed as a string and triggers a notice.
+	 *
+	 * @ticket 66138
+	 *
+	 * @covers ::parse
+	 *
+	 * @expectedIncorrectUsage WP_Block_Parser::parse
+	 */
+	public function test_parses_stringable_document_as_string() {
+		$markup   = '<!-- wp:paragraph --><p>Hello</p><!-- /wp:paragraph -->';
+		$document = new class( $markup ) {
+			private $markup;
+
+			public function __construct( $markup ) {
+				$this->markup = $markup;
+			}
+
+			public function __toString() {
+				return $this->markup;
+			}
+		};
+
+		$p = new WP_Block_Parser();
+
+		$this->assertSame( $p->parse( $markup ), $p->parse( $document ) );
+	}
+
 	/**
 	 * Ensures that implicitly closing a final open block does not repeat content parsed before it.
 	 *