Parsing php.ini values - "option --d doesn't allow an argument Fatal error: Command failed"

[ntwb]

I'm trying to parse a PHP constant to grunt-phpunit using options d:

Specifically I am trying to add this:
<const name="WP_TESTS_MULTISITE" value="1" /> (Source)

My Gruntfile.js looks like this:

phpunit: {
    'default': {
        dir: TESTS_DIR + 'phpunit/testcases/',
        configuration: 'phpunit.xml',
    },
    multisite: {
        dir: TESTS_DIR + 'phpunit/',
        configuration: TESTS_DIR + 'phpunit/multisite.xml',
        d: WP_TESTS_MULTISITE[1],
    },
    options: {
        bin: "phpunit",
        bootstrap: TESTS_DIR + 'phpunit/bootstrap.php',
    }
},

I'm always getting the following error:
option --d doesn't allow an argument Fatal error: Command failed:

The PHPUnit docs Setting PHP INI settings, Constants and Global Variables

Per the PHPUnit docs Command-Line switches I should be able to do:
-d key[=value]

I could be getting the syntax incorrect but have tried various combinations to no avail.