You're right, it's definitely a problem with the XS code.

Program received signal SIGSEGV, Segmentation fault.
doContent (parser=0x824c7a0, startTagLevel=0, enc=0x401c2bc0, 
    s=0x824e3d1 "</prefix>\n</encmap>\n", end=0x824e3e5 "", nextPtr=0x
+0)
    at xmlparse.c:1709
1709    xmlparse.c: No such file or directory.
(gdb)
[download]

The fact that it's only segfaulting on exit, and not immediately makes me think its munging the stack, but I don't have the xmlparse source in front of me, so it's hard to tell.