mysqli - PHP Fatal error: Call to a member function bind_param() on a non-object

I have the following
code:

$statement =
            $mysqli->prepare("INSERT INTO `paypal_transactions` (`txn_id`, `payer_email`,
            `mc_gross`, `mc_currency`, `expires`, `userid`) VALUES (?, ?, ?, ?, " . (time() +
            2678400) . ", ?)");

file_put_contents('error.txt',
            $mysqli->error . mysqli_error($mysqli));
$statement->bind_param('ssdsi',
            $txn_id, $payer_email, $payment_amount, $payment_currency,
            $userid);
$statement->execute();

error.txt
is blank every single time, and this is what I see in the error_log
file:

[02-Jul-2013 09:08:15
            America/Denver] PHP Fatal error: 
Call to a member function bind_param() on a
            non-object in /home4/site/public_html/paypal.php on line
            96

which is
referring to the block of code above.

I am at my
wits end with this, I have been trying to fix it for hours and it just won't work. I
cannot find any problems with my sql query and I am losing my mind trying to figure out
what's wrong.