This fix is small, less than 1KB.  The test case for the fix is well-documented.  The preamble does not document the actual changes made for the fix -- however, only 2 lines of code in MessageSet class >> parse:toClassAndSelector: were touched:
--changed "methodRef ifNil:" to "methodRef isNil ifTrue:"  
--added "methodRef isEmpty ifTrue: [^ csBlock value: nil value: nil]."

I re-created the test case that Ned describes by executing the following DoIt and then clicking around in the resulting TimeProfileBrowser:
  TimeProfileBrowser onBlock: [20 timesRepeat:  [Transcript show: 100 factorial printString]]

No walkback appeared. I ran SLint on MessageSet; no problems were reported for the [FIX] changes.