Helpme

Hello, i maded login page, but when i try to login, it say "Login Failed, Invalid Input!". In error_log: "PHP Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in /home2/minerdog/public_html/ChatPhp/login.php on line 27". my code: " if(mysqli_num_rows($query)==0){ $_SESSION['msg'] = "Login Failed, Invalid Input!"; header('location: index.php'); } " Help me