<===> input.scss
test {
  err: 2px - 2px*2px;
}
<===> error
Error: 2px and 4px*px have incompatible units.
  ,
2 |   err: 2px - 2px*2px;
  |        ^^^^^^^^^^^^^
  '
  input.scss 2:8  root stylesheet

<===> error-libsass
Error: Incompatible units: 'px*px' and 'px'.
        on line 2:8 of input.scss
>>   err: 2px - 2px*2px;
   -------^