Bulb setting on e520
I don't believe you're doing anything wrong.
The camera is taking a dark frame after your first exposure, and using that to subtract the electronic noise from your long-exposure image. I'm afraid that's exactly how it works.
If you turn off Noise Reduction (vice Noise Filter), it'll let you take another shot, but your images will be very noisy.
|Common Flashwing by digimania|
from canon dslr
|The Marilyn impersonator by Lee8282|
from Blowing in the Wind (Nature)