There is a common custom amongst many to recite the words
אל מלך נאמן
before Kriyas Shema when there is no minyan. However, in all Chabad siddurim I have davened from, this phrase is not there. Why is this?
The First Chabad Rebbe, known as the Alter Rebbe (and also known as "The Rav (Ba'al HaTanya)", the author of Shulchan Aruch Harav) writes in his Siddur:
יחיד המתפלל לעצמו יחזור תיבות אני ה' אלהיכם
An individual praying alone should repeat the words Ani Hashem Eloheichem
This is to exclude minhag ashkenaz (and what he writes in his Shulchan Aruch) that an individual says "אל מלך נאמן".