mathematical_evaluation adopter is not handling invalid equations like four plus xyz

[AhmadManzoor]

what is eight plus x
Traceback (most recent call last):
File "chatbot.py", line 22, in
response = chatbot.get_response(inputtext)
File "/home/abubakar/.local/lib/python3.6/site-packages/chatterbot/chatterbot.py", line 139, in get_response
response = self.generate_response(input_statement, additional_response_selection_parameters)
File "/home/abubakar/.local/lib/python3.6/site-packages/chatterbot/chatterbot.py", line 173, in generate_response
if adapter.can_process(input_statement):
File "/home/abubakar/.local/lib/python3.6/site-packages/chatterbot/logic/mathematical_evaluation.py", line 33, in can_process
response = self.process(statement)
File "/home/abubakar/.local/lib/python3.6/site-packages/chatterbot/logic/mathematical_evaluation.py", line 59, in process
mathparse.parse(expression, language=self.language.ISO_639.upper())
File "/home/abubakar/.local/lib/python3.6/site-packages/mathparse/mathparse.py", line 279, in parse
return evaluate_postfix(postfix)
File "/home/abubakar/.local/lib/python3.6/site-packages/mathparse/mathparse.py", line 211, in evaluate_postfix
a = stack.pop()
IndexError: pop from empty list

[AhmadManzoor]

this is my code
and i entered

what is eight plus x

from chatterbot import ChatBot
from chatterbot.trainers import ChatterBotCorpusTrainer, ListTrainer

# Create a new chat bot named Charlie
chatbot = ChatBot('casino',
	read_only=True,
	default_response= 'I am sorry, but I do not understand.',
    maximum_similarity_threshold = 0.95,
	logic_adapters=[
            'chatterbot.logic.BestMatch',
            'chatterbot.logic.TimeLogicAdapter',
        	'chatterbot.logic.MathematicalEvaluation'
    ])

trainer = ChatterBotCorpusTrainer(chatbot)
trainer.train('/home/abubakar/Desktop/casino.yml')
trainer.train("chatterbot.corpus.english")
inputtext=input('what do you want to know: ')
for i in range(100):

	response = chatbot.get_response(inputtext)
	# print(response)
	inputtext=input(response)