InAppProducts Update returns 500 Internal Server Error

[Ronny Roktel]

While developing in Java with AndroidPublisher I'm trying to update an InAppProduct object.
first of all I get the object using:

AndroidPublisher.Inappproducts.Get get = inappproducts.get(packageName, skuName);
InAppProduct inAppProduct = get.execute();

then i manipulate it according to my logic
and try to update the object using:

AndroidPublisher.Inappproducts.Update update = inappproducts.update(packageName,skuName,inAppProduct);
update.execute();

and I receive the following error message:

com.google.api.client.googleapis.json.GoogleJsonResponseException: 500 Internal Server Error
{
"code" : 500,
"message" : null
}

any ideas why is this happening? and how can I solve it?