为什么在我的python代码中没有定义'yes'

Question

print("Hello, welcome to OCRTunes")
account=input("Would you like to create an account?")
if account=='yes':
    print("Okay, just a few more steps until you have an account")
else:
    print("Okay, thank you for visiting OCRTunes")

第3行说我运行代码时没有定义'是'，但我不明白为什么，请有人帮助我