Pandas custom function randomly returns empty cells [duplicate]

Hi I have the following df with a duration column where I would like to create another 'duration_category' column based on duration bins

duration   
24  
74      
2 
17
26      
.....
120 
37
43

I have written the following function

   def return_duration_category(duration): 
    cat = ''

    if (duration > 0 and duration <=12):
        cat = '0-12'
    elif (duration > 12 and duration <= 24):
        '13-24'
    elif (duration > 24 and duration <= 36):
        cat ='25-36'
    elif (duration > 36 and duration <= 48):
        '37-48'
    elif (duration > 48 and duration <= 60):
        cat ='49-60'
    elif (duration > 60):
        cat ='60+'
    return (cat)

and applied it to the df in this fashion

df['duration_category'] = rentals_add_friction['duration'].apply(lambda x: return_duration_category(x))

The outpout works for some values, but returns empty cells for others and it seems to be quite random. I don't understand how some cells are empty as the conditions in the if statement should include all values. Can anyone help?

Output is like this:

duration      duration category  
    24         
    74         60+
    2          0-12
    17         
    26         25-36
    .....
    120        60+
    37        
    43