IndentationError expected an indented block

Question

this is my code 

stemming = PorterStemmer()

corpus = []

for i in range (0,len(df)):

s1 = re.sub('[^a-zA-Z]',repl = ' ',string = df['Email'][i])

s1.lower()

s1 = s1.split()

s1 = [stemming.stem(word) for word in s1 if word not in set(stopwords.words('english'))]

s1 = ' '.join(s1)

corpus.append(s1)

corpus[50]

 I got the error in s1 = re.sub('[^a-zA-Z]',repl = ' ',string = df['Email'][i])

how can I fix it ?