diff --git a/src/wp-includes/l10n/class-wp-translation-file.php b/src/wp-includes/l10n/class-wp-translation-file.php
index e6dbdb85be..e550f9c2f0 100644
--- a/src/wp-includes/l10n/class-wp-translation-file.php
+++ b/src/wp-includes/l10n/class-wp-translation-file.php
@@ -247,7 +247,7 @@ abstract class WP_Translation_File {
 	 * @param string $header Plural-Forms header string.
 	 * @return string Plural forms expression.
 	 */
-	protected function get_plural_expression_from_header( $header ) {
+	protected function get_plural_expression_from_header( string $header ): string {
 		if ( preg_match( '/^\s*nplurals\s*=\s*(\d+)\s*;\s+plural\s*=\s*(.+)$/', $header, $matches ) ) {
 			return trim( $matches[2] );
 		}