'local' is a reserved word in zsh 5.1 but not in earlier versions [1].
Therefore, under zsh older than 5.1, quoting is required.
This manifested as random «builtin=''» in emitted to the terminal, and
commands (such as 'echo') highlighted as errors (in red).
[1] https://github.com/zsh-users/zsh/blob/master/README#L46
(the section "Incompatibilites between 5.0.8 and 5.1")