'Solution Owner cannot be NULL' when trying to add a new solution

'Solution Owner cannot be NULL' when trying to add a new solution

When I try to add a new solution, I keep getting this error:

Solution Owner cannot be NULL (code 4832)

The URL I use to add a new solution: 
https://desk.zoho.com/api/json/solutions/addrecords?authtoken=XXXXXXXX&portal=XXXXXXXX&department=XXXXXXXX&xml=<requests><row no="1"><fl val="Solution Title">Vanuit de website</fl><fl val="Answer">Het antwoord</fl><fl val="Topic">Vanuit de website</fl><fl val="Status">Draft</fl><fl val="Solution Owner">Sample</fl></row></requests>