Stylus css prepending 'ks-' to css rem unit. Why?

Question

I'm trying to write a simple function to generate utility classes with Stylus:

spacer-maker($class, $property, $steps = 12)

  for $i in 1..$steps

    if $i < 5
      $step = .25
      $multiplier = $i
    else
      $step = 1
      $multiplier = $i - 3

    $value = $multiplier * $step

    .{$class}-{$i}
      {$property}: $value rem

spacer-maker('mt', 'margin-top')

This is working as expected EXCEPT the unit value rendered is ks-rem not rem:

.mt-1 {
  margin-top: 0.25 ks-rem;
}

I can't see any reason for this to happen, and have been trying all matter of workarounds to just output rem correctly but cannot find a working solution.

I'm using webpack driven compilation but have not had any other Stylus compilation issue to date.