Indent region in python script

I have some lines of code in a python script opened under Py Outl mode:

last=-1
for i in range(len(page_nos)-1, -1, -1):
    if page_nos[i] != 'NULL':
        last = i
        break

if last == -1:                                                                                
    page_nos[0] = 1
    last = 0

I want to put them into a function, so I add a line of function definition in front of them:

def fillnull(page_nos):

last=-1
for i in range(len(page_nos)-1, -1, -1):
    if page_nos[i] != 'NULL':
        last = i
        break

if last == -1:                                                                              
    page_nos[0] = 1
    last = 0

I then want to indent the original lines so that they can become the body of the function. By selecting the region and M-x indent-region, the region becomes:

def fillnull(page_nos):

    last=-1
    for i in range(len(page_nos)-1, -1, -1):
        if page_nos[i] != 'NULL':
            last = i
            break

        if last == -1: 
            page_nos[0] = 1
            last = 0

Why does it put extra indents in front of the second if block?

How can I indent the original lines correctly? Thanks.