POST request returns 500 error, however backend works fine- angular

So i'm currently trying to set up a post request to allow users to upload comments via my Angular API however, I'm getting a 500 error. I have tested the backend API POST request on postman and it works completely fine. This is the error I'm getting: "POST http://localhost:5000/api/v1.0/events/undefined/discussions/undefined/replies 500 (INTERNAL SERVER ERROR)"

Bellow is PostReply in the Web Service:

postReply(reply: any) {
let postData = new FormData();
postData.append('username', reply.username);
postData.append('comment', reply.comment);

return this.http.post(
'http://localhost:5000/api/v1.0/events/' + this.eventID + '/discussions/' + this.discussionId + '/replies',
postData
);


The onSubmit function:

onSubmit() {
this.webService.postReply(this.replyForm.value)
.subscribe((response: any) => {
this.replyForm.reset();
this.replies = this.webService.getReplies(this.route.snapshot.params['id'],this.route.snapshot.params['_id']);
});

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