ESP8266 MQTT not subscribing

I'm trying to follow this tutorial on connecting to AWS:

https://medium.com/accenture-the-dock/esp8266-aws-iot-core-guide-c640f2622a51

I'm getting the ESP8266 to connect to the internet but it seems like I can't MQTT to subscribe. There is another forum post on this error that I'm getting but the solution to that post doesn't work in this situation. Here is the error in the serial window:

load 0x4010f000, len 3584, room 16 
tail 0
chksum 0xb0
csum 0xb0
v2843a5ac
~ld
⸮⸮connecting to wifi

connected
3068 - conn: 1 - (46984)
Fatal exception 28(LoadProhibitedCause):
epc1=0x40212184, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000000, depc=0x00000000

--------------- CUT HERE FOR EXCEPTION DECODER ---------------

Exception (28):
epc1=0x40212184 epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000

EDIT: After a comment someone left, I decoded the stack trace and got this message for the error:

Exception 28: LoadProhibited: A load referenced a page mapped with an attribute that does not permit loads
PC: 0x40212184: strnlen_P at /home/earle/src/esp-quick-toolchain/repo/newlib/newlib/libc/sys/xtensa/string_pgmspace.c line 51
EXCVADDR: 0x00000000

Here is the part of the code this is referencing:

void callback(char* topic, byte* payload, unsigned int length)