String out of range error

[Salah71]

I am trying get this code for my HS class. I don't know much about python, i know the basics. We are trying to create strategy in Prisoners Dilemma. I got most of mine working, i just need this part to work, and it looks like its not. I keep getting an error about this highlighted line. ERROR name 

Error:
String index out of range

team_name = 'The name the team gives to itself' # Only 10 chars displayed.
strategy_name = 'The name the team gives to this strategy'
strategy_description = 'How does this strategy decide?'
ctest = 0
btest = 0
def move(my_history, their_history, my_score, their_score):
''' Arguments accepted: my_history, their_history are strings.
my_score, their_score are ints.

Make my move.
Returns 'c' or 'b'. 
'''
ctest = 0
btest = 0
if len(their_history) > 15:
for k in their_history:
if k == 'c':
ctest += 1
if k == 'b':
btest += 1
elif len(my_history)==0:
return 'c'
elif len(their_history) > 4:
if their_history[:4] == 'cbcb':
return 'b'
elif len(their_history) > 1:
if their_history[-1] == 'b' and their_history[-2] == 'b':
return 'b'
elif len(their_history) > 15:
endtest = 0
endtest = ctest/btest
if endtest >= 0.65: <-----------------------------
return 'b'
elif endtest <= 0.45: <-----------------------------
return 'c'
else:
return 'c'
# my_history: a string with one letter (c or b) per round that has been played with this opponent.
# their_history: a string of the same length as history, possibly empty. 
# The first round between these two players is my_history[0] and their_history[0].
# The most recent round is my_history[-1] and their_history[-1].

# Analyze my_history and their_history and/or my_score and their_score.
# Decide whether to return 'c' or 'b'.