Ble examples on nRF51822 - cant add trace

OK Got the ble examples and compile the ble heart rate and it works cool stuff BUT
just adding one line

Serial dbg(p9, p11);

will cause the app to fail when it gets connected - bizzare

NB I don’t actually do any debug and the app runs and advertises, only when a connection occurs does it crash.

I tried yt debug and when I connect it prints…

Program received signal SIGTRAP, Trace/breakpoint trap.
_exit (status=1) at C:/Users/peter/ble-examples/BLE_HeartRate/yotta_modules/mbed-drivers/source/retarget.cpp:568
568 __BKPT(0);

I have found out that RawSerial doesn’t cause the same grief so I can now print out some debug messages.