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]
```
const uint256& tx_hash = tx->GetHash();
    CAmount max_raw_tx_fee = ::maxTxFee;
    if (!request.params[1].isNull() && request.params[1].get_bool()) {
        max_raw_tx_fee = 0;
    }
    UniValue result(UniValue::VARR);
    UniValue result_0(UniValue::VOBJ);
    result_0.pushKV("txid", tx_hash.GetHex());
    CValidationState state;
    bool missing_inputs;
    bool test_accept_res;
    {
        LOCK(cs_main);
        test_accept_res = AcceptToMemoryPool(mempool, state, std::move(tx), &missing_inputs,
            nullptr , false , max_raw_tx_fee,  true);
    }
    result_0.pushKV("allowed", test_accept_res);
    if (!test_accept_res) {
        if (state.IsInvalid()) {
            result_0.pushKV("reject-reason", strprintf("%i: %s", state.GetRejectCode(), state.GetRejectReason()));
        } else if (missing_inputs) {
            result_0.pushKV("reject-reason", "missing-inputs");
        } else {
            result_0.pushKV("reject-reason", state.GetRejectReason());
        }
    }
    result.push_back(std::move(result_0));
    return result;
}
static std::string WriteHDKeypath(std::vector<uint32_t>& keypath)