xsi:nil="true" makes the whole element disappear

I want to send a null value using xsi:nil="true":

<mccCode>5192</mccCode>
<mrr xsi:nil="true"/>
<nameOnCardHolderStatement>Test</nameOnCardHolderStatement>

When I sent this my RAW looks like this:

<mccCode>5192</mccCode>
<nameOnCardHolderStatement>Test</nameOnCardHolderStatement>

With the mrr field missing.

When I try this with the free version it does work.

Anyone?