Kernel panic when overlaying UART TX and RX pins on nrf52833_nrf52820

When I do an overlay like this:

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&pinctrl {
uart0_default: uart0_default {
group1 {
psels = <NRF_PSEL(UART_TX, 0, 31)>,
<NRF_PSEL(UART_RX, 0, 30)>;
};
};
};
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

I get kernel panic:

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[00:00:00.000,000] <err> os: ARCH_EXCEPT with reason 4
[00:00:00.000,000] <err> os: r0/a1: 0x00000004 r1/a2: 0x00000070 r2/a3: 0x00000009
[00:00:00.000,000] <err> os: r3/a4: 0x00000000 r12/ip: 0x00000010 r14/lr: 0x00003535
[00:00:00.000,000] <err> os: xpsr: 0x6100000b
[00:00:00.000,000] <err> os: Faulting instruction address (r15/pc): 0x00008632
[00:00:00.000,000] <err> os: >>> ZEPHYR FATAL ERROR 4: Kernel panic on CPU 0
[00:00:00.000,000] <err> os: Fault during interrupt handling
[00:00:00.000,000] <err> os: Current thread: 0x200021b0 (unknown)
[00:00:00.000,000] <err> fatal_error: Resetting system
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

If I set the overlay back to 6 & 8 it works.

If I set TX/RX to pin 11/13, I get 11/8.

Are there some restrictions here that I am not aware of?

Rgds Tage