guard against bad input on numeric conversions
guard against bad input on numeric conversions

pauldardeau/python-concurrent-disk-io,pauldardeau/python-concurrent-disk-io,pauldardeau/python-concurrent-disk-io,pauldardeau/python-concurrent-disk-io,pauldardeau/python-concurrent-disk-io,pauldardeau/python-concurrent-disk-io,pauldardeau/python-concurrent-disk-io