What is the error

Question

def highlight_word(sentence, word):
lst = []
lst = sentence.split()
if word in lst:
idx = lst.index(word)
lst[idx] = lst[idx].upper()
new += ' '.join(lst)
return new

print(highlight_word("Have a nice day", "nice"))
print(highlight_word("Shhh, don't be so loud!", "loud"))
print(highlight_word("Automating with Python is fun", "fun"))

Answer 1

The line : new += ' '.join(lst). Remove the + operator
new = ' '.join(lst)

Answer 2

@Justus thanks for help