Error: $map: null is not a map for 'map-get'

I’m running Sass 3.4.7 (Selective Steve) through gulp-ruby-sass. I have a _variables.scss file with the following:

$palettes: (
    grey: (
        base:       rgb(95, 96, 98),
        mid-light:  rgb(127, 128, 129),
        light:      rgb(159, 160, 161),
        x-light:    rgb(205, 205, 205),
        xx-light:   rgb(230, 230, 230)
    ),
    purple: (
        base: rgb(70, 22, 22),
        light: rgb(110, 33, 34)
    )
);

@function palette($palette, $tone: 'base') {
    @return map-get(map-get($palettes, $palette), $tone);
}

When I compile, I get the following error:

Error: $map: null is not a map for 'map-get'
    on line 16 of /var/folders/s5/919v_t0n4v590754yhdyddy0000gn/T/gulp-ruby-sass/base/_variables.scss

I’m lost as to why this is happening.