Using Function gives ' Object of Type None type has no len()'.

[Shivam23Thaman]

I am trying to use functions to solve this but I am getting the above stated error, Please help. Here's my code :
`android_final = []
ios_final = []
def free_apps(dataset , index):
for app in dataset:
price = app[index]
if (price == 0) and (dataset == English_only_apps_android):
return android_final.append(app)
elif (price == 0) and (dataset == English_only_apps_ios):
return ios_final.append(app)

android_useful = free_apps(English_only_apps_android,7)
ios_useful = free_apps(English_only_apps_ios,4)

def datasize():
length_useful_android = len(android_useful)
length_useful_ios = len(ios_useful)
print("Length of Useful Data from the Android Dataset : ", length_useful_android)
print("\nLength of Useful Data from the IOS Dataset : ", length_useful_ios)

datasize()
`