Why do we need to quote # in ${(#):-2\#1011010} ?

[Stephane Chazelas <stephane@xxxxxxxxxxxx>]

$ echo ${:-2#1011010}
2#1011010

That # is fine when not quoted there, so why:

$ echo ${(#):-2#1011010}

$ echo ${(#):-2\#1011010}
Z

?

It seems to discard everything after the expansion within the word:

$ echo before${(#):-2#1011010}"hey, where did that go?" other args
before other args

-- 
Stephane