In Aristotle's Logic Darapti is a valid figure.
I suspect that the issue is with the so-called "existential import" :
From a modern standpoint, [we infer] "Some monsters are chimeras" from [...] "All chimeras are monsters"; but the former is often construed as implying in turn "There is something which is a monster and a chimera", and thus that there are monsters and there are chimeras. In fact, this simply points up something about Aristotle's system: Aristotle in effect supposes that all terms in syllogisms are non-empty.
In "modern term" ∀x(Fx → Gx) and ∀x(Fx → ¬Gx) are both true when ¬∃xFx is true, that is, when there are no Fs. (These are the so-called “vacuously true” universal generalizations.) So ‘All Fs are Gs’, on the modern reading, does not imply
that there are Fs, and so does not imply that some Fs are Gs.
We can see :
Historically, “Aristotelian” and “modern” logicians disagree about the validity of some syllogism forms. They disagree because of differing policies about allowing empty terms (general terms that don’t refer to any existing beings).
Compare these two arguments:
All cats are animals. Therefore : Some animals are cats.
All unicorns are animals. Therefore : Some animals are unicorns.
The first seems valid while the second seems invalid. Yet both have the same form [...]. What’s going on here?
When we read the first argument, we tend to presuppose that there’s at least one cat. Given this as an assumed additional premise, it follows validly that some animals are cats. When we read the second argument, we don’t assume that there’s at least one unicorn. Without this additional assumption, it doesn’t follow that some animals are unicorns.
The Aristotelian view, which assumes that each general term in a syllogism refers to at least one existing being, calls the argument “valid.” The modern view, which allows empty terms like “unicorn” that don’t refer to existing beings, calls the argument “invalid.”
Consider this pair of arguments with the same form (a form that’s valid on the Aristotelian view but invalid on the modern view):
All cats are mammals. All cats are furry. Therefore : Some mammals are furry.
All square circles are squares. All square circles are circles. Therefore : Some squares are circles.
The first inference is sensible, because there are cats. The second inference isn’t
sensible, because there are no square circles. Some logic books use the Aristotelian view, but most use the modern view. It makes a difference in very few cases.