python 3.4: TypeError: required field "arg" missing from arg

```python
from black_magic.decorator import wraps
def foo(*a):
    pass
wraps(foo)(foo)
```
gives: 
```
Traceback (most recent call last):
  File "k.py", line 7, in <module>
    wraps(foo)(foo)
  File "/home/thomas/src/project/black_magic/black_magic/decorator.py", line 296, in <lambda>
    return lambda wrapper, signature=signature: wraps(function, wrapper, signature)
  File "/home/thomas/src/project/black_magic/black_magic/decorator.py", line 300, in wraps
    return ASTorator.from_function(function, signature=signature)(wrapper)
  File "/home/thomas/src/project/black_magic/black_magic/decorator.py", line 237, in decorate
    code = compile(expr, filename, 'exec')
TypeError: required field "arg" missing from arg
```