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() > 3)
        throw std::runtime_error(
            RPCHelpMan{"converttopsbt",
                "\nConverts a network serialized transaction to a PSBT. This should be used only with createrawtransaction and fundrawtransaction\n"
                "createpsbt and walletcreatefundedpsbt should be used for new applications.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX,  false,  "", "The hex string of a raw transaction"},
                    {"permitsigdata", RPCArg::Type::BOOL,  true,  "false", "If true, any signatures in the input will be discarded and conversion.\n"
                            "                              will continue. If false, RPC will fail if any signatures are present."},
                    {"iswitness", RPCArg::Type::BOOL,  true,  "depends on heuristic tests", "Whether the transaction hex is a serialized witness transaction.\n"
                            "                              If iswitness is not present, heuristic tests will be used in decoding. If true, only witness deserializaion\n"
                            "                              will be tried. If false, only non-witness deserialization will be tried. Only has an effect if\n"
                            "                              permitsigdata is true."},
                },
                RPCResult{
                            "  \"psbt\"        (string)  The resulting raw transaction (base64-encoded string)\n"
                },
                RPCExamples{
                            "\nCreate a transaction\n"
                            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\":\\\"myid\\\",\\\"vout\\\":0}]\" \"[{\\\"data\\\":\\\"00010203\\\"}]\"") +
                            "\nConvert the transaction to a PSBT\n"
                            + HelpExampleCli("converttopsbt", "\"rawtransaction\"")
                },