diff --git a/lib/compat/wordpress-5.9/theme.php b/lib/compat/wordpress-5.9/theme.php
index 43862e6bf6117a..7bfe11f604c5b6 100644
--- a/lib/compat/wordpress-5.9/theme.php
+++ b/lib/compat/wordpress-5.9/theme.php
@@ -16,3 +16,13 @@ function wp_is_block_theme() {
 			is_readable( get_theme_file_path( '/templates/index.html' ) );
 	}
 }
+
+/**
+ * Note: We have to maintain this function for backward compatibility with WP 5.8.
+ * The `validate_theme_requirements` method is using `gutenberg_is_fse_theme` in older versions of WP.
+ *
+ * @return boolean Whether the current theme is a block theme or not.
+ */
+function gutenberg_is_fse_theme() {
+	return wp_is_block_theme();
+}