Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 54729)
+++ src/wp-includes/formatting.php	(working copy)
@@ -1601,7 +1601,7 @@
 
 		// Unicode sequence normalization from NFD (Normalization Form Decomposed)
 		// to NFC (Normalization Form [Pre]Composed), the encoding used in this function.
-		if ( function_exists( 'normalizer_normalize' ) ) {
+		if ( function_exists( 'normalizer_normalize' ) && class_exists( 'Normalizer' ) ) {
 			if ( ! normalizer_is_normalized( $string, Normalizer::FORM_C ) ) {
 				$string = normalizer_normalize( $string, Normalizer::FORM_C );
 			}