SASS mixin for Pixels to EM

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.5)
// Compass (v1.0.0.alpha.18)
// ----

/* PIXELS to EM */
// eg. for a relational value of 12px write em(12) when the parent is 16px
// if the parent is another value say 24px write em(12, 24)

// @include em(12);
@function em($pxval, $base: $em-base) {
    @if not unitless($pxval) {
        $pxval: strip-units($pxval);
    }
    @if not unitless($base) {
        $base: strip-units($base);
    }
    @return ($pxval / $base) * 1em;
}

// widget table headers
section header h1 {
	font-family: LatoRegular;
	font-size: em(18);
}

SassMeister-output.css

Undefined variable: "$em-base".