diff --git a/src/wp-includes/blocks.php b/src/wp-includes/blocks.php
index 787ba44d2c..9dcd193214 100644
--- a/src/wp-includes/blocks.php
+++ b/src/wp-includes/blocks.php
@@ -263,7 +263,7 @@ function register_block_style_handle( $metadata, $field_name, $index = 0 ) {
 		if ( $is_core_block ) {
 			$rtl_file = str_replace( "{$suffix}.css", "-rtl{$suffix}.css", $style_path_norm );
 		} else {
-			$rtl_file = str_replace( ".css", "-rtl.css", $style_path_norm );
+			$rtl_file = str_replace( '.css', '-rtl.css', $style_path_norm );
 		}
 
 		if ( is_rtl() && file_exists( $rtl_file ) ) {