Error: Invalid params, response status code: 500

[SeanHai]

when I use the node-bitcoin to access the running bitcoin-core node, I get this error. I use a loop to request 100 or more blocks through the JSON-RPC api.

The bitcoind.conf is:
server=1 daemon=1 rpcthreads=10 txindex=1 rpcuser=bitcoin rpcpassword=bitcoin rpctimeout=3600
The main code is :
`var bitcoin = require('bitcoin');
var client = new bitcoin.Client({
host: 'localhost',
port: 8332,
user: 'bitcoin',
pass: 'bitcoin'
});
var fs = require("fs");
fs.readFile('block_num', function (err, data) {
if (err) {
return console.error(err);
}
var i = Number(data)
var k = i+100
for (i ; i < k; i++){
client.getBlockHash(i, function(err, hash, resHeaders) {
if (err) return console.log(err);

            client.getBlock(hash, (2), function(err, block){
                    if (err) return console.log(err);
                    fs.appendFile('blocks/blk'), JSON.stringify(block,null,2)+"\n",  function(err) {
                            if (err) {
                            return console.error(err)
                            }
                            })
                    })
            })
    }
    fs.writeFile('block_num', i , function (err) {
          if (err) {
             return console.error(err);
            }
    })

});
The error is:{ Error: Invalid params, response status code: 500
at IncomingMessage. (/home/sean/Downloads/node-bitcoin/node_modules/bitcoin/lib/jsonrpc.js:110:17)
at IncomingMessage.emit (events.js:187:15)
at endReadableNT (_stream_readable.js:1094:12)
at process._tickCallback (internal/process/next_tick.js:63:19) code: -32602 }
`

How can I fix it? thanks.