Appending to a list

What is the logic behind this error:

   (14:32) gp > my(L=List([3,4,5])); L[4]=12; L
     ***   at top-level: my(L=List([3,4,5]));L[4]=12;L
     ***                                      ^--------
     ***   nonexistent component: index > 3

**Why** should I use listput() for this operation?  It is clear that
write access to L[#L+2] should be disabled; but writing to L[#L+1]
seems to be completely unambiguous — and a logical thing to do…

I do not see many situations where this would help to catch bugs too…

Puzzled,
Ilya