Gulp, Typescript and Uglify: Error: File is not in the source map

Question

With an application I'm currently developing I am also learning Gulp and using source maps. The application is a hybrid of Typescript and Javascript. The files are concatenated and minified. When passing the files through gulp-uglify an error occurs:

\src\App\node_modules\gulp-uglify\node_modules\vinyl-sourcemaps-apply\node_modules\source-map\lib\source-map\source-map-consumer.js:415
  throw new Error('"' + aSource + '" is not in the SourceMap.');
        ^
Error: "Common.ts" is not in the SourceMap.
at SourceMapConsumer_sourceContentFor [as sourceContentFor] (Z:\Dev\financial-app\src\App\node_modules\gulp-uglify\node_modules\vinyl-sourcemaps-apply\node_modules\source-map\lib\source-map\source-map-consumer.js:415:13)
at SourceMapGenerator.<anonymous> (Z:\Dev\financial-app\src\App\node_modules\gulp-uglify\node_modules\vinyl-sourcemaps-apply\node_modules\source-map\lib\source-map\source-map-generator.js:233:42)
at Array.forEach (native)

This only happens when using gulp-sourcemaps. When I remove the source map generation calls in the pipeline the issue doesn't occur but then I don't have any of the source maps either.

return merge(
        // TS compile
        gulp.src(filePath.appjs.src.ts)
          .pipe(sourcemaps.init())
          .pipe(ts(tsProject))
          .pipe(sourcemaps.write()),

        // JS compile
        gulp.src(filePath.appjs.src.js)
    )
    .pipe(sourcemaps.init({loadMaps: true}))
    .pipe(concat('appscripts.js'))
    .pipe(size({ title: 'APPJS' }))
    .pipe(sourcemaps.write())
    .pipe(gulp.dest(filePath.appjs.dest))
    //.pipe(uglify(filePath.uglifyOptions))
    .pipe(concat('appscripts.min.js'))
    .pipe(size({ title: 'APPJS minified' }))
    .pipe(gulp.dest(filePath.appjs.dest));

Perhaps useful to know, the typescript is to be found in ./js/ts/ and the javascript in ./js/App.