Wireless power is provided to a WPP-compliant wireless device by generating a first radio frequency (RF) signal at a first frequency. The transmitter circuit is inductively coupled to the compliant wireless device using the first RF signal. A second RF signal is generated at a second frequency. The presence of a WPP-noncompliant wireless device is detected by detecting a third RF signal at a third frequency that is a harmonic of the second frequency. The non-compliant wireless device is protected by reducing, in response to detecting the third RF signal, a signal strength for the first RF signal.