looks like zsh doesn't correctly parse this:
$ zsh -c 'echo $((`:`))'
zsh:1: bad math expression: illegal character: \M-]

a little whitespace makes it happy:
$ zsh -c 'echo $(( `:`))'
0
$ zsh -c 'echo $((`:` ))'
0

same for $():
$ zsh -c 'echo $(($(:)))'
zsh:1: bad math expression: illegal character: \M-]
$ zsh -c 'echo $(( $(:)))'
0
$ zsh -c 'echo $(($(:) ))'
0

looks like it's related to the subshell not outputting anything.
if you use 'echo 0' instead of ':', it works out.

$ zsh --version
zsh 5.0.7 (x86_64-pc-linux-gnu)
-mike