TypeError: 'generator' object is not callable

I have a generator defined like this:

def lengths(x):
    for k, v in x.items():
        yield v['time_length']

And it works, calling it with

for i in lengths(x):
    print i

produces:

3600
1200
3600
300

which are the correct numbers.

However, when I call it like so:

somefun(lengths(x))

where somefun() is defined as:

def somefun(lengths):
    for length in lengths():  # <--- ERROR HERE
        if not is_blahblah(length): return False

I get this error message:

TypeError: 'generator' object is not callable

What am I misunderstanding?