PHP-problem: Varför funkar inte detta?

<?php

// Koppla upp mot databasen
mysql_connect("localhost","det","xxxxxx");
@mysql_select_db("databas") or die( "Unable to select database");
mysql_query("SET NAMES 'utf8'");

$trimmed = "äpple";

$query = mysql_query("SELECT * FROM livsmedel WHERE Namn LIKE '%".$trimmed."%'");

$numrows=mysql_num_rows($query);

echo "$numrows";

?>

Får detta felmeddelande:
Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /usr/www/script.php on line 12

PHP klagar på denna rad:
$numrows=mysql_num_rows($query);

Vad kan vara felet?