Yep grab the sentence, not the two words separately. From the docs:
Do keep in mind that some keywords are harder to distinguish from others, and especially keywords with only one syllable (like ‘One’) might lead to false-positives (e.g. when you say ‘Gone’). This is the reason that Apple, Google and Amazon all use at least three-syllable keywords (‘Hey Siri’, ‘OK, Google’, ‘Alexa’). A good one would be “Hello world”.