Stack walking in MMTk not correctly identifying not yet obsolete methods?

During the stack trace in:

http://jikesrvm.anu.edu.au/cattrack/results/rvmx86lnx32.anu.edu.au/perf.954/production/default/perf-dacapo/xalan-3GC/3/Output.txt

we get a CMID that when loaded from the compiled methods array turns out to be null. This indicates that we didn't calculate the set of active CMIDs correctly during GC and we've snipped the compiled method believing it to be obsolete.