zsh: substitute-if-undef-or-null for $1, $2 …: ${1:substitution} doesn't work

I'd like to have standard parameters, i.e., my minimum reproducible example is:

#!/usr/bin/zsh
a=${1:a}
printf 'a: "%s"\n' "${a}"
b=${2:./build}
printf 'b: "%s"\n' "${b}"

I'd expect to see, when running ./demo.zsh without arguments:

a: "a"
b: "./build"

Instead I get

a: ""
./demo.zsh:4: bad floating point constant

- why does ${1:a} seem to assume $1 to be set and non-zero, but the $a expanded is empty string? - where does the floating point misparsing come from? If I replace ./build with build, it complains about b being an unknown modifier. If I run ./demo.zsh asdf bar, I get

a: "/tmp/fasf"
./demo.zsh:4: bad floating point constant

which, frankly is even more confusing; where does /tmp/ come from (it's the cwd).