How jasmine spyOn a generic method

I try to make a spy on a generic method in typescript,
but cannot get Jasmine to recognise it.

---

I have code

http: HttpClient <- Not proper code, just showing type.
...
this.http.get<Customer[]>(url);

where I want to mock the get<...> method.

const httpMock = {} as HttpClient;
spyOn(httpMock, 'get')
   .and.returnValue(of({} as Customer[]));

But when I run the test I get

Error: : get() method does not exist
Usage: spyOn(, )