[Logi error]

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