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() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            RPCHelpMan{"sendrawtransaction",
                "\nSubmits raw transaction (serialized, hex-encoded) to local node and network.\n"
                "\nAlso see createrawtransaction and signrawtransactionwithkey calls.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX,  false,  "", "The hex string of the raw transaction"},
                    {"allowhighfees", RPCArg::Type::BOOL,  true,  "false", "Allow high fees"},
                },
                RPCResult{
            "\"hex\"             (string) The transaction hash in hex\n"
                },
                RPCExamples{
            "\nCreate a transaction\n"
            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\" : \\\"mytxid\\\",\\\"vout\\\":0}]\" \"{\\\"myaddress\\\":0.01}\"") +
            "Sign the transaction, and get back the hex\n"
            + HelpExampleCli("signrawtransactionwithwallet", "\"myhex\"") +
            "\nSend the transaction (signed hex)\n"
            + HelpExampleCli("sendrawtransaction", "\"signedhex\"") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("sendrawtransaction", "\"signedhex\"")
                },
            }.ToString());
    std::promise<void> promise;
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VBOOL});