indexing in format strings

Fri Apr 11 17:33:02 EDT 2014

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.