     
     func testUpdateInvoice() throws {
         do {
-            let object = try drop?.stripe?.invoices.update(
+            let object = try self.drop?.stripe?.invoices.update(
                 invoice: self.invoiceId,
                 description: "Update Invoice"
             ).serializedResponse()

    
    func testFetchingInvoiceItems() throws {
        do {
            let object = try drop?.stripe?.invoices.listItems(forInvoice: self.invoiceId)
            XCTAssertNotNil(object)
        } catch let error as StripeError {
            switch error {

    
    func testFetchUpcomingInvoice() throws {
        do {
            let object = try self.drop?.stripe?.invoices.upcomingInvoice(forCustomer: self.customerId).serializedResponse()
            XCTAssertNotNil(object)
        } catch let error as StripeError {
            switch error {
