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?