Move to first row with OpenSpout

I read an xlsx- or csv-file with OpenSpout and want to read the first row (column-titles) before further processing. I get the rowIterator like this:

$iterator = $reader->getSheetIterator()->current()->getRowIterator();

The key is at 0 at the beginning. I try to move the key to 1 with

$iterator->next();

but I get the error
Fatal error: Uncaught Error: Data must be loaded before reading…

What am I missing here?