     
     func testListAllInvoices() throws {
         do {
-            let object = try drop?.stripe?.invoices.listAll().serializedResponse()
+            let object = try self.drop?.stripe?.invoices.listAll().serializedResponse()
             XCTAssertNotNil(object)
         } catch let error as StripeError {
             switch error {

    
    func testCreatingInvoice() throws {
        do {
            let object = try drop?.stripe?.invoices.create(
                forCustomer: self.customerId,
                subscription: self.subscriptionId
            ).serializedResponse()

    
    func testFetchingInvoice() throws {
        do {
            let object = try self.drop?.stripe?.invoices.fetch(invoice: self.invoiceId).serializedResponse()
            XCTAssertNotNil(object)
        } catch let error as StripeError {
            switch error {
