Db.fresh_key compilation error

eroor

Error
File "main.opa", line 3, characters 8-18, (3:8-3:18 | 40-50)
Expression has a record type incompatible for access to field fresh_key. Its
type, limited to the most probable fields you want to access, is
{ build: !for all 'a 'b 'c . Db.builder('a, 'b, 'c) -> Db.path('a, 'b, 'c);
  exists: !for all 'd 'e 'f . Db.path('d, 'e, 'f) -> bool;
  get_engine: !for all 'g 'h 'i . Db.path('g, 'h, 'i) -> 'i;
  get_val: !for all 'j 'k . Db.ref_path('j, 'k) -> Db.val_path('j, 'k);
  read: !for all 'l 'm 'n . Db.path('l, 'm, 'n) -> 'm;
  remove: !for all 'o 'p . Db.ref_path('o, 'p) -> void;
  write: !for all 'q 'r . Db.ref_path('q, 'r), 'q -> {  } }.
Hint:
  Perhaps you meant remove or read ?

main.opa

database intmap(string) /users

key = Db.fresh_key(@/users)