Running master 4d661ba:
```
src/bitcoin-cli combinepsbt [""] or src/bitcoin-cli combinepsbt []
```

```
  * frame #0: 0x000000010016054a bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional_detail::optional_base<CMutableTransaction>::is_initialized(this=0x0000000000000000) const at optional.hpp:394 [opt]
    frame #1: 0x000000010016054a bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional_detail::optional_base<CMutableTransaction>::optional_base(this=<unavailable>, rhs=0x0000000000000000) at optional.hpp:196 [opt]
    frame #2: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional<CMutableTransaction>::optional(this=0x000070000b9fe7c0, (null)=0x0000000000000000) at optional.hpp:958 [opt]
    frame #3: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional<CMutableTransaction>::optional(this=0x000070000b9fe7c0, (null)=0x0000000000000000) at optional.hpp:958 [opt]
    frame #4: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(this=0x000070000b9fe7c0, psbt_in=0x0000000000000000) at sign.h:576 [opt]
    frame #5: 0x0000000100142ae5 bitcoind`combinepsbt(JSONRPCRequest const&) [inlined] PartiallySignedTransaction::PartiallySignedTransaction(this=0x000070000b9fe700, psbt_in=<unavailable>) at sign.h:576 [opt]
    frame #6: 0x0000000100142ae0 bitcoind`combinepsbt(request=<unavailable>) at rawtransaction.cpp:1526 [opt]
    frame #7: 0x0000000100170f0a bitcoind`CRPCTable::execute(this=<unavailable>, request=0x000070000b9fead0) const at server.cpp:557 [opt]
```
}.ToString());
    RPCTypeCheck(request.params, {UniValue::VARR}, true);
    
    std::vector<PartiallySignedTransaction> psbtxs;
    UniValue txs = request.params[0].get_array();
    for (unsigned int i = 0; i < txs.size(); ++i) {
        PartiallySignedTransaction psbtx;
        std::string error;
        if (!DecodePSBT(psbtx, txs[i].get_str(), error)) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
        }
        psbtxs.push_back(psbtx);
    }
    PartiallySignedTransaction merged_psbt(psbtxs[0]); 
    
    for (auto it = std::next(psbtxs.begin()); it != psbtxs.end(); ++it) {
        if (*it != merged_psbt) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "PSBTs do not refer to the same transactions.");
        }
        merged_psbt.Merge(*it);
    }
    if (!merged_psbt.IsSane()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Merged PSBT is inconsistent");
    }
    UniValue result(UniValue::VOBJ);
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << merged_psbt;
    return EncodeBase64((unsigned char*)ssTx.data(), ssTx.size());
}
UniValue finalizepsbt(const JSONRPCRequest& request)