indexing in format strings

[blindanagram]

With:

   l = [1,2,3]

this:

   print('{0[0]:d}..{0[2]:d}'.format(l))

gives 1..3 but this:

   print('{0[0]:d}..{0[-1]:d}'.format(l))

gives:

   Traceback (most recent call last):
    File "<string>", line 1, in <fragment>
   builtins.TypeError: list indices must be integers, not str

which seems to me counterintuitive.

I expected indexing in a format string to behave as it does elsewhere
but this seems not to be true.