I set up a process filter, killed the only process matching the filter, and then pressed "k" again. To my surprise, htop core dumped. I tried it again and again, and it happened every time. I'm guessing this bug is caused by a missing check that there is actually a process currently selected.

The output is:

htop 1.0.1 aborting. Please report bug at http://htop.sf.net
Backtrace: 
htop[0x407d3f]
/lib64/libc.so.6[0x3303a32ac0]
htop(main+0x1e7e)[0x40b2ce]
/lib64/libc.so.6(__libc_start_main+0xfd)[0x3303a1ec9d]
htop[0x404f89]
[1]    28568 abort (core dumped)  htop