"XML Parsing Error: no root element found" Ionic/Angular - json

Question

There are many topics with this similar error: "XML Parsing Error: no root element found", but none of them cover the error for Ionic framework.
I am writing an app with Ionic framework, and this is the function to POST the data:
http.ts
postData(url, body) {
const headers = new Headers();
headers.append('Content-Type', 'application/json');
const result = this.http.post(url, JSON.stringify(body), {headers: headers})
.map(response => {
console.log(response.json());
response.json();
})
.subscribe(data => {
console.log(typeof(data));
console.log("subscribe data");
},
error => {
console.log(error.statusText);
})
A button on my app's HTML form calls the onSubmit function to send the data like below.
submit.ts
onSubmit() {
var submitData = {
'customer': {
'phone_number': this.clientInfo.phone,
'name': this.clientInfo.name
},
'amount': this.clientInfo.amount
}
const submitResult = this.httpProvider.postData(postTransactionsURL, submitData);
Now, on Firefox, this is the error that is shown in the console:
XML Parsing Error: no root element found
Location: http://localhost:3000/transactions
Line Number 1, Column 1:
As reported in other posts, the error does not show up on e.g. Edge, but nevertheless, there is still a more generic error.
Is there anything I could try to resolve this? I would assume the header is correct for this?
Update: added POST network information from console:
Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 0
Date: Fri, 02 Mar 2018 12:42:36 GMT
X-Powered-By: Express
Update2: server.js response
app.post('/transactions', (req, res) => {
......
......
......
res.status(200).send();
} else {
res.status(400).send("Invalid transaction");
}
});