Deepsleep wake reason crashes board when on battery power

[EndoMonkey]

Hi,

When I place code in the main.py to check the reason for wake from deepsleep (deepsleep shield) the program crashes with the error message below when on battery power:

rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
ets Jun 8 2016 00:22:57

rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
ets Jun 8 2016 00:22:57

rst:0x10 (RTCWDT_RTC_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff9028,len:8
load:0x3fff9030,len:1060
load:0x4009fa00,len:0
ho 12 tail 0 room 4
load:0x4009fa00,len:14084
entry 0x400a05d4
Traceback (most recent call last):
File "main.py", line 50, in <module>
File "/flash/lib/deepsleep.py", line 157, in get_wake_status
File "/flash/lib/deepsleep.py", line 81, in peek
IndexError: bytes index out of range
MicroPython v1.8.6-849-baa8c33 on 2018-01-29; LoPy with ESP32
Type "help()" for more information.

The program works fine when powered via the USB port and only crashes on battery power!

Firmware v. 1.15.0.b1 and deepsleep.py v. 1.0.5.

Is this a known issue or could I be doing something odd in my code?

Any thoughts much appreciated!