sortByKey() will fail on empty RDD

>>> sc.parallelize(zip(range(4), range(0)), 5).sortByKey().count()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/davies/work/spark/python/pyspark/rdd.py", line 532, in sortByKey
    for i in range(0, numPartitions - 1)]
IndexError: list index out of range
>>>