Yes you can get an invalid test if the control window dosent work. If you follow the instructions it will say that if there is no line in the control window it means the test hasnt worked and is invalid and you will need to re-test so off you go to get another test and try again and let us know!!!
When I tested early, I always used first response!! Good Luck!!
Yes, it is possible to get an invalid test. Sometimes a defective test just gets out there. I have used all the cheap brands though and never gotten a wrong result. I have used Walmart's brand and Walgreens brand. Both said I was pregnant when I was pregnant. So, I would just get another test!!! Good luck!!