diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index d4331ae3706ae..c947609a35698 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -98,32 +98,31 @@ function wp_enqueue_block_custom_css() {
  * } $block
  */
 function wp_render_custom_css_class_name( $block_content, $block ) {
-	$class_name_attr = $block['attrs']['className'] ?? null;
-
-	if ( ! is_string( $class_name_attr ) || ! str_contains( $class_name_attr, 'wp-custom-css-' ) ) {
+	$class_name_attr   = $block['attrs']['className'] ?? null;
+	$class_name_prefix = 'wp-custom-css-';
+	if ( ! is_string( $class_name_attr ) || ! str_contains( $class_name_attr, $class_name_prefix ) ) {
 		return $block_content;
 	}
 
 	// Parse out the 'wp-custom-css-*' class name added by wp_render_custom_css_support_styles().
-	$custom_class_name = null;
-	$token_delimiter   = " \t\f\r\n";
-	$class_token       = strtok( $class_name_attr, $token_delimiter );
+	$matched_class_name = null;
+	$token_delimiter    = " \t\f\r\n";
+	$class_token        = strtok( $class_name_attr, $token_delimiter );
 	while ( false !== $class_token ) {
-		if ( str_starts_with( $class_token, 'wp-custom-css-' ) ) {
-			$custom_class_name = $class_token;
+		if ( str_starts_with( $class_token, $class_name_prefix ) ) {
+			$matched_class_name = $class_token;
 			break;
 		}
 		$class_token = strtok( $token_delimiter );
 	}
-	if ( null === $custom_class_name ) {
+	if ( null === $matched_class_name ) {
 		return $block_content;
 	}
 
 	$tags = new WP_HTML_Tag_Processor( $block_content );
-
 	if ( $tags->next_tag() ) {
 		$tags->add_class( 'has-custom-css' );
-		$tags->add_class( $custom_class_name );
+		$tags->add_class( $matched_class_name );
 	}
 
 	return $tags->get_updated_html();
diff --git a/src/wp-includes/block-supports/elements.php b/src/wp-includes/block-supports/elements.php
index 374da09e8bc8c..54b96aa1dc064 100644
--- a/src/wp-includes/block-supports/elements.php
+++ b/src/wp-includes/block-supports/elements.php
@@ -237,22 +237,43 @@ function wp_render_elements_support_styles( $parsed_block ) {
  * @see wp_render_elements_support_styles
  * @since 6.6.0
  *
- * @param  string $block_content Rendered block content.
- * @param  array  $block         Block object.
- * @return string                Filtered block content.
+ * @param string $block_content Rendered block content.
+ * @param array  $block         Block object.
+ * @return string               Filtered block content.
+ *
+ * @phpstan-param array{
+ *     attrs: array{
+ *         className?: string,
+ *         ...
+ *     },
+ *     ...
+ * } $block
  */
 function wp_render_elements_class_name( $block_content, $block ) {
-	$class_string = $block['attrs']['className'] ?? '';
-	preg_match( '/\bwp-elements-\S+\b/', $class_string, $matches );
+	$class_name_attr   = $block['attrs']['className'] ?? null;
+	$class_name_prefix = 'wp-elements-';
+	if ( ! is_string( $class_name_attr ) || ! str_contains( $class_name_attr, $class_name_prefix ) ) {
+		return $block_content;
+	}
 
-	if ( empty( $matches ) ) {
+	// Parse out the 'wp-elements-*' class name.
+	$matched_class_name = null;
+	$token_delimiter    = " \t\f\r\n";
+	$class_token        = strtok( $class_name_attr, $token_delimiter );
+	while ( false !== $class_token ) {
+		if ( str_starts_with( $class_token, $class_name_prefix ) ) {
+			$matched_class_name = $class_token;
+			break;
+		}
+		$class_token = strtok( $token_delimiter );
+	}
+	if ( null === $matched_class_name ) {
 		return $block_content;
 	}
 
 	$tags = new WP_HTML_Tag_Processor( $block_content );
-
 	if ( $tags->next_tag() ) {
-		$tags->add_class( $matches[0] );
+		$tags->add_class( $matched_class_name );
 	}
 
 	return $tags->get_updated_html();
diff --git a/tests/phpunit/tests/block-supports/wpRenderElementsSupport.php b/tests/phpunit/tests/block-supports/wpRenderElementsSupport.php
index 9103fcba90b66..007ba8312e495 100644
--- a/tests/phpunit/tests/block-supports/wpRenderElementsSupport.php
+++ b/tests/phpunit/tests/block-supports/wpRenderElementsSupport.php
@@ -105,6 +105,60 @@ public function test_elements_block_support_class( $color_settings, $elements_st
 		);
 	}
 
+	/**
+	 * Tests that a non-string `className` attribute does not cause a fatal
+	 * error and the block content is returned unmodified.
+	 *
+	 * Block attributes such as `className` are always expected to be strings,
+	 * however invalid stored data can result in other types being present. The
+	 * render filter should fail gracefully rather than passing an array to
+	 * `preg_match()`.
+	 *
+	 * @ticket 65379
+	 *
+	 * @covers ::wp_render_elements_class_name
+	 */
+	public function test_elements_block_support_class_with_non_string_class_name(): void {
+		$block = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => array( '0', '1' ),
+			),
+		);
+
+		$block_content = "<p class=\"0 1\">Test</p>\n";
+
+		$this->assertSame(
+			$block_content,
+			wp_render_elements_class_name( $block_content, $block ), // @phpstan-ignore argument.type (Intentionally passing bad attrs array.)
+			'Block content should be returned unchanged when className is not a string'
+		);
+	}
+
+	/**
+	 * Tests that a 'my-wp-elements-*' class name is skipped from processing.
+	 *
+	 * @ticket 65379
+	 *
+	 * @covers ::wp_render_elements_class_name
+	 */
+	public function test_elements_block_support_class_with_invalid_elements_prefix(): void {
+		$block = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'my-wp-elements-foo',
+			),
+		);
+
+		$block_content = "<p>Test</p>\n";
+
+		$this->assertSame(
+			$block_content,
+			wp_render_elements_class_name( $block_content, $block ),
+			'Block content should be returned unchanged when className lacks a class with the expected prefix'
+		);
+	}
+
 	/**
 	 * Data provider.
 	 *