I want to execute get_supply from currency.hpp.

Here is the definition:

asset get_supply( symbol_name symbol )const {
        accounts t( _contract, symbol );
        return t.get(symbol).balance;
     }

Currency constructor:

currency( account_name contract )
         :_contract(contract)
         { }

I am trying like that:

void get_supply(extended_asset mytoken) {
        extended_symbol es = mytoken.get_extended_symbol();
        auto sym = es.symbol;
        auto curr = currency(N(mytoken.token));
        auto stats = curr.get_supply(sym);
    }

mytoken.token -> custom token contract (copy of eosio.token)

I am getting error:

Error 3050003: eosio_assert_message assertion failure Error Details: assertion failure with message: unable to find key

Anyone knows how to use this function correctly?