Bash: Error `Unbound variable' when testing unset variable

Problem

I have defined strict bash settings:

set -o errexit  # Exit on error
set -o nounset  # Trigger error when expanding unset variables

But now I get the error $1: Unbound variable when I execute this line:

 [ $# -eq 0 -o "$1" = --help ] && showUsage()

Use the `[[' compound command for more intelligent parsing, without any `unbound' error:

[[ $# == 0 || $1 == --help ]] && showUsage()

This works because $1 == --help is only evaluated if the first expression $# == 0 equals false.

Use parameter expansion for positional parameter $1:

[ $# -eq 0 -o "${1:-unset}" = --help ] && showUsage()

From info bash:

`${PARAMETER:-WORD}'

If PARAMETER is unset or null, the expansion of WORD is substituted. Otherwise, the value of PARAMETER is substituted.