find module of `fromJust Nothing'

[Serge D. Mechveliani]

Who can advise, please, about an error message 
                                       Nat1: Maybe.fromJust: Nothing
?

Probably, this is a bug in my program, and first, I need to locate it.
The head test module is  Nat1.hs.  The output is

--------------------------------------------------------------------
...
...

--(ProofSearchState ----------------------------------
tree =
(formula1 Any
 [(1, NegCp (spent cost 1283),
   (formula1 Any []))
  (2, ArC fromList [(n,n_c)],
   (formula6 Any
    [(1, NegCp (spent cost 683),
      (formula6 Any []))]))
  (3, Ind n,
   (formula1
    [(1, IndMember,
Nat1: Maybe.fromJust: Nothing
-----------------------------------------------------------------------

-- and break.
By the last line GHC says, probably, that executing the module  Nat1  
caused the call of  `fromJust Nothing'. 

And  Nat1.hs  does not contain any call of  fromJust,  neither even the 
substring of "fromJust".  
Nat1  imports many other modules.
Why GHC does not mention some other module from which this
`fromJust Nothing' has directly appeared?

Thank you in advance for your explanations,

-----------------
Serge Mechveliani
mechvel at botik.ru