diff --git src/wp-includes/ms-functions.php src/wp-includes/ms-functions.php
index 3eb7f67..214cc3c 100644
--- src/wp-includes/ms-functions.php
+++ src/wp-includes/ms-functions.php
@@ -801,8 +801,8 @@
 	$from_name = get_site_option( 'site_name' ) == '' ? 'WordPress' : esc_html( get_site_option( 'site_name' ) );
 	$message_headers = "From: \"{$from_name}\" <{$admin_email}>\n" . "Content-Type: text/plain; charset=\"" . get_option('blog_charset') . "\"\n";
 
-	$user = get_user_by( 'login', $user );
-	$switched_locale = switch_to_locale( get_user_locale( $user ) );
+	$_user = get_user_by( 'login', $user );
+	$switched_locale = switch_to_locale( get_user_locale( $_user ) );
 
 	$message = sprintf(
 		/**
@@ -896,8 +896,8 @@
 	if ( ! apply_filters( 'wpmu_signup_user_notification', $user, $user_email, $key, $meta ) )
 		return false;
 
-	$user = get_user_by( 'login', $user );
-	$switched_locale = switch_to_locale( get_user_locale( $user ) );
+	$_user = get_user_by( 'login', $user );
+	$switched_locale = switch_to_locale( get_user_locale( $_user ) );
 
 	// Send email with activation link.
 	$admin_email = get_site_option( 'admin_email' );