SysTick timer on nRF52 fires 1/5 as often as expected

Hi,

I'm trying to use the SysTick timer on nRF52 to time some various tasks for debugging. I configured it like so, which I expected would cause SysTick_Handler to be called every 1ms (SystemCoreClock is indeed 64000000):

SysTick_Config(SystemCoreClock/1000);

However, if I simply increment a counter in the SysTick_Handler and print out the value every 1s (I'm using app_timer to fire a timer every 1000ms), the counter only increments by 200 every second, instead of 1000.

Has anyone been able to use SysTick successfully on nRF52? Note that this is with PCA10036 - I looked in the errata but didn't see any issues related to SysTick_Config.