diff --git a/packages/block-library/src/post-excerpt/index.php b/packages/block-library/src/post-excerpt/index.php
index a9dde84d801482..cd6d5746a239e0 100644
--- a/packages/block-library/src/post-excerpt/index.php
+++ b/packages/block-library/src/post-excerpt/index.php
@@ -84,8 +84,7 @@ function register_block_core_post_excerpt() {
  * Returns 100 because 100 is the max length in the setting.
  */
 if ( is_admin() ||
-	defined( 'REST_REQUEST' ) ||
-	'REST_REQUEST' ) {
+	defined( 'REST_REQUEST' ) && REST_REQUEST ) {
 	add_filter(
 		'excerpt_length',
 		function() {