zsh associative array value data types

In ZSH, how do you set the value of an associative array parameter to an array?

e.g.,

% declare -A a=()
% a[first]=(1 2)
zsh: a: attempt to set slice of associative array

And when I try and append the associative array a with the array b it seems to work but silently converts the value of b from an array to a scalar.

% declare -A a=()
% declare -a b=(1 2)
% print ${(t)b}
array
% a+=( [b]=($b) )
% print $a[b]
(1 2)
% print ${#a[b]}
5
% for ((i=1; i<=${#a[b]}; i++)) ; do print -n "${a[b][$i]}" ; done ; echo
(1 2)
%