SASS error: Incompatible units: 'px' and 'px*px'

input.scss

// ----
// Sass (v3.4.21)
// Compass (v1.0.3)
// ----

@mixin font-size($base: 16 ){
  font-size: $base + px; //fallback for old browsers
  font-size: (.0625 * $base) + rem;
}
$font-size-base:          14 !default;
$line-height-base:        1.846 !default;
$input-line-height:       floor(($font-size-base * $line-height-base)) !default;
$input-padding-y:         6px !default;
$input-height: (($font-size-base * $input-line-height) + ($input-padding-y * 2)) !default;


div {
    height:$input-height;
  /*fonts with variable*/
  font-size: ($font-size-base * 1px);
  font-size: ($font-size-base * .0625rem);
  /*fonts with mixin*/
  @include font-size()
}

output.css

div {
  height: 362px;
  /*fonts with variable*/
  font-size: 14px;
  font-size: 0.875rem;
  /*fonts with mixin*/
  font-size: 16px;
  font-size: 1rem;
}

http://stackoverflow.com/questions/42797624/sass-error-incompatible-units-px-and-pxpx/42797952#42797952