I'm getting a PDF generation error with biserver-3.5RC2.

Basic report, with a PNG or GIF in the report header.
Report renders in HTML correctly.

I can run the exact same report in PRD 3.5RC2 and the PDF generates.
I can run the same report without an image in the header, and biserver generates the PDF.

I tried saving the PDF and I got an HTML page instead.

What am I doing wrong?