diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index 20f15e015bfca..32eb56823a90c 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -791,6 +791,9 @@ protected static function sanitize( $input, $valid_block_names, $valid_element_n
 	 * @return string The new selector.
 	 */
 	protected static function append_to_selector( $selector, $to_append, $position = 'right' ) {
+		if ( ! str_contains( $selector, ',' ) ) {
+			return 'right' === $position ? $selector . $to_append : $to_append . $selector;
+		}
 		$new_selectors = array();
 		$selectors     = explode( ',', $selector );
 		foreach ( $selectors as $sel ) {