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]
```
{
    if (request.fHelp || request.params.size() < 2 || request.params.size() > 4)
        throw std::runtime_error(
            RPCHelpMan{"signrawtransactionwithkey",
                "\nSign inputs for raw transaction (serialized, hex-encoded).\n"
                "The second argument is an array of base58-encoded private\n"
                "keys that will be the only keys used to sign the transaction.\n"
                "The third optional argument (may be null) is an array of previous transaction outputs that\n"
                "this transaction depends on but may not yet be in the block chain.\n",
                {
                    {"hexstring", RPCArg::Type::STR,  false,  "", "The transaction hex string"},
                    {"privkeys", RPCArg::Type::ARR,  false,  "", "A json array of base58-encoded private keys for signing",
                        {
                            {"privatekey", RPCArg::Type::STR_HEX,  false,  "", "private key in base58-encoding"},
                        },
                        },
                    {"prevtxs", RPCArg::Type::ARR,  true,  "null", "A json array of previous dependent transaction outputs",
                        {
                            {"", RPCArg::Type::OBJ,  true,  "", "",
                                {
                                    {"txid", RPCArg::Type::STR_HEX,  false,  "", "The transaction id"},
                                    {"vout", RPCArg::Type::NUM,  false,  "", "The output number"},
                                    {"scriptPubKey", RPCArg::Type::STR_HEX,  false,  "", "script key"},
                                    {"redeemScript", RPCArg::Type::STR_HEX,  true,  "omitted", "(required for P2SH or P2WSH) redeem script"},