SARI does not work for short sequence?

Hi, 

Thanks for the great work! 
I have a problem with SARI score. Looks like it does not support sequence with one word for example? See the example below:

    test_src = "global"
    test_pred = "scenarioPattern"
    test_ref = "scenarioPattern"
    print(SARIsent(test_src, test_pred, [test_ref]))

the score is 0.17 rather than 1.

Is a bug or as expected?

Thank you.