Bool given in di mysqli_num_rows() dan ysqli_fetch_assoc() saat login?

<form id="loginform" action="index.php?login_attempt=1" method="post">
    <p class="animate4 bounceIn"><input type="text" id="username" name="username" placeholder="Username" /></p>
    <p class="animate5 bounceIn"><input type="password" id="password" name="password" placeholder="Password" /></p>
    <p class="animate6 bounceIn"><button class="btn btn-default btn-block">Masuk</button></p>

</form>
<?php
if(isset($_GET['login_attempt']))
{
	$spf=sprintf("Select * from user_login where username='%s' and password='%s'",$_POST['username'],$_POST['password']);
	$rs=mysqli_query($conn, $spf);
	$rw=mysqli_fetch_array($rs);
	$rc=mysqli_num_rows($rs);

	if($rc==1)
	{
		$_SESSION['login_hash']=$rw['login_hash'];
		$_SESSION['login_user']=$rw['username'];
		echo "<script>window.location='dashboard.php'</script>";
	}
}
?>

Muncul error ini di form login. Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, bool given in C:\xampp\htdocs\inventory-ptgps\login.php on line 13

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, bool given in C:\xampp\htdocs\inventory-ptgps\login.php on line 14