Error: TezosOperationError: (branch) proto.017-PtNairob.contract.empty_transaction

Question

So, i'm doing the royalties function for my NFT Marketplace. My idea was to make a big map with the royalties (user and amount) for each new token created, and it's working fine. My problem is when i'm trying to buy a item, it's gives me this error:

TezosOperationError: (branch) proto.017-PtNairob.contract.empty_transaction

My buy function was working fine until i add the royalties function. Here is the buy entrypoint:

@sp.entry_point
def buy(self, params):
    
    k = sp.local('k', 0)
    sp.for i in self.data.data[params.token_id]:
        sp.if k.value < params.quant:
            sp.if params.seller == i.holder:
                sp.if params.price == i.amount:
                    i.holder = sp.sender
                    i.collectable = False
                    k.value += 1
    price = sp.utils.mutez_to_nat(params.price)
    total_price = params.quant * price
    

    self.fa2_transfer(self.data.token, params.seller, sp.sender, params.token_id, params.quant)
    
    total_royal = sp.local('total_royal', 0)
    sp.for g in self.data.royalties[params.token_id]:
        
        royal_to_send = total_price * (g.amount / 1000)
        
        total_royal.value += royal_to_send

        royal_to_send = sp.utils.nat_to_mutez(royal_to_send)

        sp.send(g.address, royal_to_send)
    
    
    total_price -= total_royal.value
    total_price = sp.as_nat(total_price)
    total_price = sp.utils.nat_to_mutez(total_price)
    
    sp.send(params.seller, sp.split_tokens(total_price, 90, 100))

My royalties metadata:

https://api.ghostnet.tzkt.io/v1/contracts/KT1JJf89Vmy3w3j5d5oq6TmGSucbXr3iQ5eN/bigmaps/royalties/keys

My token metadata:

https://api.ghostnet.tzkt.io/v1/contracts/KT1JJf89Vmy3w3j5d5oq6TmGSucbXr3iQ5eN/bigmaps/data/keys

=== Update

I discovered this line "sp.send(g.address, royal_to_send)" is giving error (i removed it and the transaction goes fine), but i can't find why and a solution about it. By doing some tests it's clear the address is working, but amount not. I defined amount with sp.TNat because it is a normal number.

Answer 1

empty_transaction occurs when you're sending 0 mutez with no data. That's the case when you do sp.send(receiver, sp.mutez(0)).

Your last line is probably equivalent to that in some / all cases.

You should not use nat_to_mutez or nat_to_tez as you can use the mutez type everywhere. I would start by fixing this last point before trying to fix your other problem.

Answer 2

I found the issue. The problem isn't the nat_to_mutez, but this: royal_to_send = total_price * (g.amount / 1000). g.amount / 1000 always return a float number, so, i don't know what happens in blockchain (because it don't accept float numbers), but, doing first total_price * g.amount, and after so, dividing by 1000 will work fine.