TypeError: is not a function TypeScript

I have a very simple class like:

export class Party {
    constructor(
        public id:Identifier,
        public partyName: PartyName,
        public person:Person
    ) {  }

    copy():Party {
        let copyParty = new Party(this.id, null, null);
        return copyParty;
    }
}

I want to use the copy function (or method?) in another class (specifically a service) importing this class like:

... (party => {
let copyParty:Party = party.copy();
...

But I get the following exception:

EXCEPTION: Uncaught (in promise): TypeError: party.copy is not a function

I tried let copyParty:Party = Function.call(party.copy, copy) (got some exceptions) as well as let copyParty:Party = party.copy; (returns the function definition, not the copied the object).

What am I missing here? Thanks.