macroexpansion (slime-expand-1) results in error

C-c Ret returns an error on some macros (but not on others):

The value NIL is not of the expected type ARRAY.
   [Condition of type TYPE-ERROR]

Restarts:
 0: [*ABORT] Return to SLIME's top level.
 1: [ABORT-BREAK] Reset this thread
 2: [ABORT] Kill this thread

Backtrace:
  0: (CCL::ARRAY-DATA-AND-OFFSET NIL)
  1: (MAKE-STRING-INPUT-STREAM NIL 0 0)
  2: (READ-FROM-STRING NIL T NIL :START 0 :END NIL :PRESERVE-WHITESPACE NIL)
  3: ((:INTERNAL SWANK::FROM-STRING))


As stated above on other macros it works fine.

This is on latest CCL release on 64-bit Windows 7, slime 2015-10-18.

I don't know how to generate more debugging information.  Any thoughts?

Thanks,

Mirko