We answer a question of J. Rosický and W. Tholen by showing that the class of complete Boolean algebras (which is a prereflective subcategory of the category of frames) is not an intersection of reflective subcategories of the category of frames. In order to deduce this result, we first prove the following observation (using some basic facts about congruence frames): the three-element chain belongs to every reflective subcategory of the category of frames which contains the class of complete Boolean algebras.