Testing service returning error from http.get

I have this service:

@Injectable({providedIn: 'root'})
export class CaseManagementService {

    constructor(private http: HttpService) {
    }

    getDocumentDetails(caseId: string, docId: string): Observable<DocumentDetails> {
        return this.http.get<DocumentDetails>(UrlConstants.CASES_URL + `/${caseId}/Documents/${docId}/Details`);
    }
}

and for testing purposes I have created the following jasmine test:

it('should handle error when retrieving document details', () => {
    const errorResponse = new HttpErrorResponse({ error: 'error', status: 500 });
    spyOn(service['http'], 'get').and.returnValue(of(errorResponse));

    service.getDocumentDetails('case1', 'doc1').subscribe({
        next: () => fail('expected an error, not document details'),
        error: error => expect(error.message).toContain('error')
    });

    expect(service['http'].get).toHaveBeenCalledWith(UrlConstants.CASES_URL + '/case1/Documents/doc1/Details');
});

No matter what I do I keep ending in the next section of my subscribe and it would be much appreciated if anyone can explain to me what I'm doing wrong.

You should use throwError instead of of.

import { throwError } from 'rxjs';

it('should handle error when retrieving document details', () => {
    const errorResponse = new HttpErrorResponse({ error: 'error', status: 500 });
    spyOn(service['http'], 'get').and.returnValue(throwError(() => errorResponse));

    service.getDocumentDetails('case1', 'doc1').subscribe({
        next: () => fail('expected an error, not document details'),
        error: error => expect(error.message).toContain('error')
    });

    expect(service['http'].get).toHaveBeenCalledWith(UrlConstants.CASES_URL + '/case1/Documents/doc1/Details');
});

HttpErrorResponse is not throwing an error itself and of creates a successful observable that emits a value.