Index: src/wp-includes/fonts/class-wp-font-utils.php
===================================================================
--- src/wp-includes/fonts/class-wp-font-utils.php	(revision 57565)
+++ src/wp-includes/fonts/class-wp-font-utils.php	(working copy)
@@ -132,7 +132,7 @@
 			$slug_elements
 		);
 
-		return sanitize_text_field( join( ';', $slug_elements ) );
+		return sanitize_text_field( implode( ';', $slug_elements ) );
 	}
 
 	/**