diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index 2cd7644ed54b1..e80fe0c47cdab 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -12,17 +12,28 @@
  *
  * @param array $parsed_block The parsed block.
  * @return array The same parsed block with custom CSS class name added if appropriate.
+ *
+ * @phpstan-param array{
+ *     blockName: string|null,
+ *     attrs: array{
+ *         className?: string,
+ *         style?: array{
+ *             css?: string,
+ *             ...
+ *         },
+ *         ...
+ *     },
+ *     ...
+ * } $parsed_block
  */
 function wp_render_custom_css_support_styles( $parsed_block ) {
-	$block_type = WP_Block_Type_Registry::get_instance()->get_registered( $parsed_block['blockName'] );
-
-	if ( ! block_has_support( $block_type, 'customCSS', true ) ) {
+	$custom_css = $parsed_block['attrs']['style']['css'] ?? null;
+	if ( ! is_string( $custom_css ) || '' === trim( $custom_css ) ) {
 		return $parsed_block;
 	}
 
-	$custom_css = trim( $parsed_block['attrs']['style']['css'] ?? '' );
-
-	if ( empty( $custom_css ) ) {
+	$block_type = WP_Block_Type_Registry::get_instance()->get_registered( $parsed_block['blockName'] );
+	if ( ! block_has_support( $block_type, 'customCSS', true ) ) {
 		return $parsed_block;
 	}
 
@@ -32,9 +43,10 @@ function wp_render_custom_css_support_styles( $parsed_block ) {
 	}
 
 	// Generate a unique class name for this block instance.
-	$class_name         = wp_unique_id_from_values( $parsed_block, 'wp-custom-css-' );
-	$updated_class_name = isset( $parsed_block['attrs']['className'] )
-		? $parsed_block['attrs']['className'] . " $class_name"
+	$class_name          = wp_unique_id_from_values( $parsed_block, 'wp-custom-css-' );
+	$existing_class_name = $parsed_block['attrs']['className'] ?? null;
+	$updated_class_name  = is_string( $existing_class_name )
+		? "$existing_class_name $class_name"
 		: $class_name;
 
 	_wp_array_set( $parsed_block, array( 'attrs', 'className' ), $updated_class_name );
@@ -76,9 +88,22 @@ function wp_enqueue_block_custom_css() {
  * @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_custom_css_class_name( $block_content, $block ) {
-	$class_string = $block['attrs']['className'] ?? '';
+	$class_string = $block['attrs']['className'] ?? null;
+
+	if ( ! is_string( $class_string ) || '' === $class_string ) {
+		return $block_content;
+	}
+
 	preg_match( '/\bwp-custom-css-\S+\b/', $class_string, $matches );
 
 	if ( empty( $matches ) ) {
diff --git a/src/wp-includes/blocks.php b/src/wp-includes/blocks.php
index cc1ac60667773..6a6418d966457 100644
--- a/src/wp-includes/blocks.php
+++ b/src/wp-includes/blocks.php
@@ -2604,9 +2604,9 @@ function unregister_block_style( $block_name, $block_style_name ) {
  * @since 5.8.0
  * @since 6.4.0 The `$feature` parameter now supports a string.
  *
- * @param WP_Block_Type $block_type    Block type to check for support.
- * @param string|array  $feature       Feature slug, or path to a specific feature to check support for.
- * @param mixed         $default_value Optional. Fallback value for feature support. Default false.
+ * @param WP_Block_Type|null $block_type    Block type to check for support.
+ * @param string|array       $feature       Feature slug, or path to a specific feature to check support for.
+ * @param mixed              $default_value Optional. Fallback value for feature support. Default false.
  * @return bool Whether the feature is supported.
  */
 function block_has_support( $block_type, $feature, $default_value = false ) {