I kind of suck at boolean algebra, but:
https://github.com/bitcoin/bitcoin/blob/37612099ec7314b15a07d8bac55161ed4e8e7491/src/rpc/rawtransaction.cpp#L1672


Unless I'm mistaken (which is highly likely) the whole thing can't work  (except the edge case where `permitsigdata=true` and you have no sig data) 
{
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 3)
        throw std::runtime_error(
                            "converttopsbt \"hexstring\" ( permitsigdata iswitness )\n"
                            "\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"
                            "\nArguments:\n"
                            "1. \"hexstring\"              (string, required) The hex string of a raw transaction\n"
                            "2. permitsigdata           (boolean, optional, default=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.\n"
                            "3. iswitness               (boolean, optional) 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.\n"
                            "\nResult:\n"
                            "  \"psbt\"        (string)  The resulting raw transaction (base64-encoded string)\n"
                            "\nExamples:\n"
                            "\nCreate a transaction\n"
                            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\":\\\"myid\\\",\\\"vout\\\":0}]\" \"[{\\\"data\\\":\\\"00010203\\\"}]\"") +
                            "\nConvert the transaction to a PSBT\n"
                            + HelpExampleCli("converttopsbt", "\"rawtransaction\"")