<no source> seen in generated site map

[neway]

I'm using gulp-sourcemaps to build a Post CSS based theme - and am seeing a "no sources" entry in the sources attribute of the generated map file.

using: "gulp-sourcemaps": "^3.0.0" & "gulp-postcss": "^10.0.0",

function styles() {
  const autoprefixer = require('autoprefixer');
  const postcss = require('gulp-postcss');
  const nested = require('postcss-nested');
  const calc = require('postcss-calc');
  const atImport = require('postcss-import');
  const perfectionist = require('perfectionist');
  const postcssCustomMedia = require('postcss-custom-media');
  const postcssCustomProperties = require('postcss-custom-properties');
  const sourcemaps = require('gulp-sourcemaps');
  const rename = require('gulp-rename');

  return gulp.src('./css/neway_mods.pcss')
    .pipe(sourcemaps.init())
    .pipe(postcss([
      atImport(),
      nested(),
      postcssCustomMedia(),
      postcssCustomProperties({ preserve: false }),
      calc(),
      autoprefixer({ grid: 'no-autoplace' }),
      perfectionist({ indentSize: 2 }),
    ]))
    .pipe(sourcemaps.mapSources(function (sourcePath, file) {
      // source paths are prefixed with '../src/'
      return '' + sourcePath;
    }))
    .pipe(rename(function (path) {
      // Updates the object in-place
      if (path.extname == '.pcss') {
        path.extname = '.css';
      }
    }))
    .pipe(sourcemaps.write('.'))
    .on('error', function (errorInfo) {  // if the error event is triggered, do something
      console.log(errorInfo.toString()); // show the error information
      this.emit('end'); // tell the gulp that the task is ended gracefully and resume
    })
    //.pipe(replace(',"<no source>"', ''))
    .pipe(gulp.dest('./css'));
}

This results in the map file:

{"version":3,"sources":["../../../../../core/themes/olivero/css/base/media-queries.pcss.css","neway_mods.pcss","<no source>"],"names":[],"mappings":"AA.....

I've been suppressing the error using a replacement clause at the end of the task - to remove the string - but thought it worthwhile noting here for any ideas?