unpaid internships and visas

<p>Hi,
Not sure if anyone can help me, but I'll try anyway...
I have an unpaid internship in Washington D.C. for this summer, but despite having lived in the US when I was younger, I'm not a US citizen and currently live in the UK. However, I'll only be in the US for 7 weeks, which is within the 90-days visa waiver limit and I'm not getting paid...but technically I will be working. Do I need a visa? I've looked at the FAQs on the US Embassy webpage and it doesn't say anything about these circumstances, and I'd rather not phone them and be on hold for 3 hours or something...so any advice would be great.
Thanks!</p>