getProp puzzle

[David Epstein]

The script of stack “A” has a getProp handler to report the number of lines in an object’s script:

getProp nSL
  return the number of lines in the script of the target
end nSL

With stack A frontmost, I type in the message box “the nSL of this stack” or “the nSL of stack’A’”, and I get the expected result.  But with stack A still frontmost, if I type in the message box “the nSL of stack ‘B’” I get an empty result (not even a “0”).  

What am I missing?

Thanks very much.

David Epstein