diff --git a/phpBB/includes/functions_privmsgs.php b/phpBB/includes/functions_privmsgs.php
index 1639eb1a4c..d7a87ca356 100644
--- a/phpBB/includes/functions_privmsgs.php
+++ b/phpBB/includes/functions_privmsgs.php
@@ -892,6 +892,12 @@ function update_unread_status($unread, $msg_id, $user_id, $folder_id)
 			AND folder_id = $folder_id";
 	$db->sql_query($sql);
 
+	// If the message is already marked as read, we just skip the rest to avoid negative PM count
+	if (!$db->sql_affectedrows())
+	{
+		return;
+	}
+
 	$sql = 'UPDATE ' . USERS_TABLE . "
 		SET user_unread_privmsg = user_unread_privmsg - 1
 		WHERE user_id = $user_id";