diff --git a/src/wp-includes/l10n.php b/src/wp-includes/l10n.php
index 1a0474f465..b7d9d24af0 100644
--- a/src/wp-includes/l10n.php
+++ b/src/wp-includes/l10n.php
@@ -138,7 +138,7 @@ function determine_locale() {
 		$determined_locale = get_user_locale();
 	}
 
-	if ( ! empty( $_GET['wp_lang'] ) && 'wp-login.php' === $GLOBALS['pagenow'] ) {
+	if ( ! empty( $_GET['wp_lang'] ) && ! empty( $GLOBALS['pagenow'] ) && 'wp-login.php' === $GLOBALS['pagenow'] ) {
 		$determined_locale = sanitize_text_field( $_GET['wp_lang'] );
 	}