Missing case in exception handling

[martinschaef]

We are experimenting with a static analysis tool and it suggested this change to make the exception handling more robust. The problem it reports is that f[0] is accessed in line 75, but in line 84 you actually check if f is empty and raise an exception.

[coveralls]

Coverage remained the same at 90.173% when pulling 9c045cc9d54425a8acea7ca124568b2673a724ea on martinschaef:patch-1 into 657b1b49da5ea1593dd7f950eae99a88a8ada87a on apache:master.