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]
```
CMutableTransaction mtx;
    if (!DecodeHexTx(mtx, request.params[0].get_str()))
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    CTransactionRef tx(MakeTransactionRef(std::move(mtx)));
    const uint256& hashTx = tx->GetHash();
    CAmount nMaxRawTxFee = maxTxFee;
    if (!request.params[1].isNull() && request.params[1].get_bool())
        nMaxRawTxFee = 0;
    { 
    LOCK(cs_main);
    CCoinsViewCache &view = *pcoinsTip;
    bool fHaveChain = false;
    for (size_t o = 0; !fHaveChain && o < tx->vout.size(); o++) {
        const Coin& existingCoin = view.AccessCoin(COutPoint(hashTx, o));
        fHaveChain = !existingCoin.IsSpent();
    }
    bool fHaveMempool = mempool.exists(hashTx);
    if (!fHaveMempool && !fHaveChain) {
        
        CValidationState state;
        bool fMissingInputs;
        if (!AcceptToMemoryPool(mempool, state, std::move(tx), &fMissingInputs,
                                nullptr , false , nMaxRawTxFee)) {
            if (state.IsInvalid()) {
                throw JSONRPCError(RPC_TRANSACTION_REJECTED, FormatStateMessage(state));
            } else {
                if (fMissingInputs) {
                    throw JSONRPCError(RPC_TRANSACTION_ERROR, "Missing inputs");
                }
                throw JSONRPCError(RPC_TRANSACTION_ERROR, FormatStateMessage(state));