Zephyr crashing when I connect to a device

Question

Vidar Berg has been helping me connect to a device and I am finally able to do that, but as soon as I do, Zephyr crashes.

This is with a lightly modified central_uart project

[00:00:00.024,810] <0x1b>[0m<inf> central_uart: Scan module initialized<0x1b>[0m␍␊

[00:00:00.024,810] <0x1b>[0m<inf> central_uart: NUS Client module initialized<0x1b>[0m␍␊

[00:00:00.029,846] <0x1b>[0m<inf> central_uart: Scanning successfully started<0x1b>[0m␍␊

[00:00:00.093,719] <0x1b>[0m<inf> central_uart: Filters matched. Address: E8:A0:DB:24:21:62 (random) connectable: 1<0x1b>[0m␍␊

[00:00:00.289,886] <0x1b>[0m<inf> central_uart: Connected: E8:A0:DB:24:21:62 (random)<0x1b>[0m␍␊

[00:00:00.290,039] <0x1b>[1;33m<wrn> central_uart: Failed to set security: -12<0x1b>[0m␍␊

[00:00:00.290,100] <0x1b>[1;31m<err> central_uart: Stop LE scan failed (err 0)<0x1b>[0m␍␊

[00:00:00.441,497] <0x1b>[0m<inf> central_uart: MTU exchange done<0x1b>[0m␍␊

ASSERTION FAIL [conn] @ WEST_TOPDIR/zephyr/subsys/bluetooth/host/gatt.c:5433␍␊

⇥ invalid parameters␍␊

␍␊

[00:00:01.791,351] <0x1b>[0m<inf> central_uart: Service discovery completed<0x1b>[0m␍␊

[00:00:01.800,994] <0x1b>[1;31m<err> os: r0/a1: 0x00000004 r1/a2: 0x00001539 r2/a3: 0x20000b60<0x1b>[0m␍␊

[00:00:01.800,994] <0x1b>[1;31m<err> os: r3/a4: 0x00000004 r12/ip: 0x00000000 r14/lr: 0x000217df<0x1b>[0m␍␊

[00:00:01.801,025] <0x1b>[1;31m<err> os: xpsr: 0x21000000<0x1b>[0m␍␊

[00:00:01.801,025] <0x1b>[1;31m<err> os: Faulting instruction address (r15/pc): 0x00031072<0x1b>[0m␍␊

[00:00:01.801,086] <0x1b>[1;31m<err> os: >>> ZEPHYR FATAL ERROR 4: Kernel panic on CPU 0<0x1b>[0m␍␊

[00:00:01.801,116] <0x1b>[1;31m<err> os: Current thread: 0x20002568 (BT RX WQ)<0x1b>[0m␍␊

[00:00:01.869,415] <0x1b>[1;31m<err> os: Halting system<0x1b>[0m␍␊

The program crashes here:

 return snprintk(str, len, "%02X:%02X:%02X:%02X:%02X:%02X (%s)",
 addr->a.val[5], addr->a.val[4], addr->a.val[3],
 addr->a.val[2], addr->a.val[1], addr->a.val[0], type);

Which is in 
addr.h (sic) Quite what code is doing in a header file is beyond me. 

The prototype for the function is this:

int bt_addr_le_to_str(const bt_addr_le_t *addr, char *str, size_t len);

The values passed look reasonable to me.
addr->a.val[0]-[5] have the correct MAC address in it
str has (in decimal)  23 0 ... in it so that looks like it's just got a single character and anyway,
it shouldn't make a difference because, surely, that's being written into. 

len = 30

type ends up with "random" in it. 

I am assuming
snprintk is based on
snprint and so shouldn't copy more than len bytes anyway. 

The string being used is that is passed into bt_addr_le_to_str is:
 char addr[BT_ADDR_LE_STR_LEN];
and BT_ADDR_LE_STR_LEN is set to 30 so that all stacks up nicely.
Can anyone give me some pointers as to what might be wrong here?
BTW  Vidar Berg that little project you sent me does not crash like this, but I can't
see where you set up the characteristics for that project, there is no reference to
BT_GATT_SERVICE_DEFINE anywhere in it. I would happily just mod that if I could work out how you've set up
its service and characteristics.
Thanks