I'm tying --interpret-error and interpret of menhir.

For this sample, I typed menhir --interpret-error parser.mly or menhir --interpret parser.mly in the command line, then for any expression I entered, it returned

File "(stdin)", line 1, characters 0-1:
Error: unexpected character.
(I believe I am reading a sentence, but may be off.)

Does anyone know what's wrong?