NRF.setSecurity fails with INVALID_PARAM

Hi!
I'm trying the various BLE Fuctions and according to the API reference, this should work:

NRF.setSecurity({passkey:"123456", mitm:1, display:0});

however, I always get this error:

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         |_| espruino.com
 2v04 (c) 2019 G.Williams
>Uncaught Error: BLE error 0x7 (INVALID_PARAM)
 at line 1 col 54
NRF.setSecurity({passkey:"123456", mitm:1, display:0});
                                                     ^

I don't have anything else in the code, just the one line. Is this somehow a documentation error or am I doing something wrong?