MSP430FR6989: Using RTC_C in calendar mode - code is locking up

Part Number:MSP430FR6989

I've ported a section of the stopwatch code into my own program (still running on MSP430FR6989).

When I call the stopWatch() function it just locks up at bis_SR_register statment.

void stopWatch()
{

// stays in LPM3 while stopwatch is running and wakes up every 10ms to update clock and LCD
__bis_SR_register(LPM3_bits | GIE); // Enter LPM3
__no_operation();

I'm new to TI parts, so don't understand how interrupts are structured. When I pause the program, I can see that no registers in RTC_C have changed. Since the timer was set up, I expect to see things incrementing.

Thanks in advance for your help.