Fehler: „Ungültige JSON-RPC-Antwort“ beim Ausführen von „Schätzgas“ auf Infura

Warum erhalte ich eine Fehlermeldung, wenn ich eine Gasschätzung auf infura durchführe. Ich benutze web3 v1.0und hier ist mein Code:

let Web3 = require('web3'); 
var web3 = new Web3(new Web3.providers.HttpProvider('http://ropsten.infura.io/key'));

const account = '0x5561E1e11199317E2186.....';
const contractAddress = '0x72A0411ee2f3.....'; 

var tokenizeContract = new web3.eth.Contract( [ ...abi... ], '0x72A0411ee2f328E34AB8DE74e51820bC257ccDE5' ); 

const contractFunction = tokenizeContract.methods.setOrder(web3.utils.toHex('location'), web3.utils.toHex('productName'), web3.utils.toHex('customer'), 3000, 2, 6000, web3.utils.toHex('timestamp'));

contractFunction.estimateGas(function(error, gasAmount) {
    if(!error) {
        console.log(gasAmount);
    } 
    else {
        console.log(error);
    }
});

Das ist der Fehler:

Error: Invalid JSON RPC response: ""
    at Object.InvalidResponse (/home/user/code/project_poc/node_modules/web3-core-helpers/src/errors.js:42:16)
    at XMLHttpRequest.request.onreadystatechange (/home/user/code/project_poc/node_modules/web3-providers-http/src/index.js:73:32)
    at XMLHttpRequestEventTarget.dispatchEvent (/home/user/code/project_poc/node_modules/xhr2/lib/xhr2.js:64:18)
    at XMLHttpRequest._setReadyState (/home/user/code/project_poc/node_modules/xhr2/lib/xhr2.js:354:12)
    at XMLHttpRequest._onHttpRequestError (/home/user/code/project_poc/node_modules/xhr2/lib/xhr2.js:544:12)
    at ClientRequest.<anonymous> (/home/user/code/project_poc/node_modules/xhr2/lib/xhr2.js:414:24)
    at ClientRequest.emit (events.js:180:13)
    at Socket.socketErrorListener (_http_client.js:395:9)
    at Socket.emit (events.js:180:13)
    at emitErrorNT (internal/streams/destroy.js:64:8)
    at process._tickCallback (internal/process/next_tick.js:178:19)