Какие артикли правильно употребить в данном случае? Take an umbrella with you, __ Dan!

Take an umbrella with you, please, Dan!

Only "an" is right. We can not use "a" in this case as the next word "umbrella" begins with "u" which is a vowel letter. And we can not use "the" as it is the first time we are talking about it and we are talking in general.