cli_wallet Invalid cast from object_type to string error

[readmagic]

When I use command

transfer initminer alice {"amount":"100000", "precision":3, "nai":"@@000000021"} "hello world" true

return

7 bad_cast_exception: Bad Cast
Invalid cast from object_type to string
    {"type":"object_type"}
    variant.cpp:478 as_string

I have compare with bitshares project.That project's wallet Api don't use complex class.
like this method.

condenser_api::legacy_signed_transaction wallet_api::transfer(
   string from,
   string to,
   condenser_api::legacy_asset amount,
   string memo,
   bool broadcast )

amount is object_type,but in variant.cpp:478 not dealwith this type.
I'm run in develop branch.