Picosat script fails with trivial zero case

[cjdrake]

$ picosat
p cnf 0 1
0
Traceback (most recent call last):
  File "...", line 137, in <module>
    sys.exit(main())
  File "...", line 87, in main
    _, nvars, clauses = ast2expr(ast).encode_cnf()
  File "...", line 773, in encode_cnf
    raise ValueError("expected a CNF expression")
ValueError: expected a CNF expression