nRF52840 SPI Slave

I am trying to use peripheral/spis as a reference for configuring my nRF52840 DK as a slave. The SPI clock frequency is 1 MHz. The SPI master is another micro-controller.

On the first call to nrf_drv_spis_buffers_set(...), I get an interrupt event NRF_DRV_SPIS_BUFFERS_SET_DONE and after that I don't get NRF_DRV_SPIS_XFER_DONE event at all.

I can see the SPI clock, MOSI data from the master and CSN low (before master starts transmitting) on the o-scope. What could possibly be wrong?