52First, for periods of residence of up to three months, Article 6 of Directive 2004/38 limits the conditions and formalities of the right of residence to the requirement to hold a valid identity card or passport and, under Article 14(1) of the directive, that right is retained as long as the Union citizen and his family members do not become an unreasonable burden on the social assistance system of the host Member State (judgment of 21 December 2011, Ziolkowski and Szeja, C‑424/10 and C‑425/10, EU:C:2011:866, paragraph 39).
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