// Check for a valid length

@function is-length($value) {
  @return contains(0 "auto" "initial" "inherit", $value)
          or type-of($value) == "number" and not unitless($value)
          or str-slice($value + "", 1, 4) == "calc";
}

// Check for a valid length

@function is-length($value) {
  @return type-of($value) != "null" and (str-slice($value + "", 1, 4) == 'calc'
       or index(auto inherit initial 0, $value)
       or (type-of($value) == "number" and not(unitless($value))));
}

Add check for null values in length function
Add check for null values in length function
