HELP! Return Statement Debugging

[HappyMan]

Dear All
I am a beginner to Python. I wrote the code below but does not understand why when I key in "0" and then a negative number (says "-5"), the print line appeared twice -- and I have no idea how the second print line (ie. "The Absolute Value of 0 is None" come into the picture. Please help.

def absoluteValue(n):
    if n < 0:
        return -n
    elif n > 0:
        return
    else:
        print("Please key in a value greater or lower than 0 !!\t ")
        main()

def main():
    y = input("Key in an integer : \t")
    n = int(y)
    print("The Absolute Value of", y, " is ", absoluteValue(n))

main()

Result :
Key in an integer : 0
Please key in a value greater or lower than 0 !!
Key in an integer : -5
The Absolute Value of -5 is 5
The Absolute Value of 0 is None