(Logic II) Existential Import of Categorical Propositions

http://www.fallacyfiles.org/existent.html

And, in more detail:

http://www.preservearticles.com/201106017320/the-existential-import-of-propositions.html

So, in sum:

Since particular propositions assume existence ("at least one..."), subalternation does not produce valid results with universal (A or E) propositions where the subject denotes an empty set, (eg., All unicorns are pink, therefore some unicorns are pink),  Arguments of this form are (on the modern, Boolean approach) only conditionally valid (i.e., on the condition that the subject term denotes an existing thing).