As the courts have interpreted it, 5A protects you if they can't even prove the safe is yours, or if otherwise admitting that you know the combination or that you know what's in the safe would result in incriminating testimony. 5A doesn't apply here because they know the device is his, and that prior to his arrest he had regular access to it. It's not incriminating testimony, it's delivery of evidence in a criminal trial subject to a lawful subpoena.