ATSAMD21 sleep + watchdog? Has anyone got sleep/standby + watchdog to work together on their Arduino Zero or Sparkfun SAMD mini?

I am able to get the WDT to reset the device, and am able to put it to sleep - but can't get the WDT to bring it out of sleep.  Every time I try, it pops right back out of sleep mode again as if SCB-&gt;SCR |= SCB_SCR_SLEEPDEEP_Msk was never even called
use this:

attachInterrupt(wakeUpPin, wakeupFunction, HIGH);
    USBDevice.detach();
    SCB-&gt;SCR |= SCB_SCR_SLEEPDEEP_Msk;
    __DSB();
    __WFI();

void wakeupFunction(){
  NVIC_SystemReset();
}