Fatal error: Call to a member function getStock() on a non-object in Mplusqapiclient.php on line 605

[dswmedia]

I am using the following code like suggested on the development portal:

// Then we initialize the variables
$branchNumber = 1;
$articleNumbers = 4542;
$amountChanged = -3;

// Then we call the getStock() function wrapped in a try/catch block to intercept any exceptions.
try {
if (false !== $mplusqapiclient->updateStock($branchNumber, $articleNumber, $amountChanged)) {
exit('Stock succesfully updated.');
} else {
exit('Unable to update stock.');
}
} catch (MplusQAPIException $e) {
exit($e->getMessage());
}

This results in:

Fatal error: Call to a member function getStock() on a non-object in Mplusqapiclient.php on line 605

Yes I am sure that i am using the right articleNumber.

Also there is a typo on the $articleNumbers vs $articleNumber