Increment error

I have 2 couchbase servers in the cluster and i get lots of errors when i increment a value:

[24-Sep-2014 12:27:29] WARNING: [pool -----] child 28757 said into stderr: “NOTICE: PHP message: PHP Fatal error: Uncaught exception ‘CouchbaseLibcouchbaseException’ with message ‘Failed to increment value in the server: Item not stored (did you try to append/prepend to a missing key?)’ in /var/www/html/incr.php:26”
[24-Sep-2014 12:27:29] WARNING: [pool -----] child 28757 said into stderr: “Stack trace:”
[24-Sep-2014 12:27:29] WARNING: [pool -----] child 28757 said into stderr: “#0 /var/www/html/incr.php(26): Couchbase->increment(‘2014-09-24_12_2…’, 1, 1, 0, 1)”
[24-Sep-2014 12:27:29] WARNING: [pool -----] child 28757 said into stderr: “#1 {main}”
[24-Sep-2014 12:27:29] WARNING: [pool -----] child 28757 said into stderr: " thrown in /var/www/html/incr.php on line 26"

And this is the code:

$cbStats = new Couchbase(‘couchbase-cluster-ip’, ‘’, ‘’, ‘stats-bucket’);

$date = date(‘Y-m-d_H’);

$cbStats->increment($date . ‘_’ . $key, 1, true, 0, 1);

Thanks