Generating bootstrap-sass source map via connect-assets is throwing an "Invalid mapping" error

[deployable]

Initially raised over here: js-kyle/connect-assets#350

When using the connect-assets css() method in dev mode, the following "Invalid mapping" error is thrown when using node-sass 4.x.

Reverting to node-sass 3.x works ok.

connect-assets can serve the css and map files directly. node-sass can generate the css and map files from the sources.

It's possibly a node-sass or source-map issue, just raising here in case.

{ Error: /sass-issue/app/views/index.pug:7
    5|     meta(name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no")
    6| 
  > 7|     != css("bootstrap")
    8|     != js("jquery")
    9|     != js("bootstrap")
    10| 

Invalid mapping: {"generated":{"line":1432,"column":28},"source":"bootstrap-sass/bootstrap/_type.scss","original":{"line":286,"column":-1},"name":null}
    at SourceMapGenerator_validateMapping [as _validateMapping] (/sass-issue/node_modules/source-map/lib/source-map-generator.js:277:13)
    at SourceMapGenerator_addMapping [as addMapping] (/sass-issue/node_modules/source-map/lib/source-map-generator.js:101:12)
    at /sass-issue/node_modules/source-map/lib/source-node.js:345:13
    at SourceNode_walk [as walk] (/sass-issue/node_modules/source-map/lib/source-node.js:224:9)
    at SourceNode_walk [as walk] (/sass-issue/node_modules/source-map/lib/source-node.js:220:13)
    at SourceNode_walk [as walk] (/sass-issue/node_modules/source-map/lib/source-node.js:220:13)
    at SourceNode_toStringWithSourceMap [as toStringWithSourceMap] (/sass-issue/node_modules/source-map/lib/source-node.js:336:8)
    at new BundledAsset (/sass-issue/node_modules/mincer/lib/mincer/assets/bundled.js:94:31)
    at Index.Base.buildAsset (/sass-issue/node_modules/mincer/lib/mincer/base.js:535:12)
    at /sass-issue/node_modules/mincer/lib/mincer/index.js:141:26
    at Index.module.exports.cacheAsset (/sass-issue/node_modules/mincer/lib/mincer/helpers/caching.js:52:12)
    at Index.buildAsset (/sass-issue/node_modules/mincer/lib/mincer/index.js:140:33)
    at Index.Base.findAsset (/sass-issue/node_modules/mincer/lib/mincer/base.js:348:15)
    at Index.findAsset (/sass-issue/node_modules/mincer/lib/mincer/index.js:114:22)
    at Environment.findAsset (/sass-issue/node_modules/mincer/lib/mincer/environment.js:137:21)
    at Assets.getAssetByPath (/sass-issue/node_modules/connect-assets/lib/assets.js:230:29)
    at /sass-issue/node_modules/connect-assets/lib/assets.js:180:22
    at eval (eval at wrap (/sass-issue/node_modules/pug-runtime/wrap.js:6:10), <anonymous>:14:46)
    at template (eval at wrap (/sass-issue/node_modules/pug-runtime/wrap.js:6:10), <anonymous>:24:98)
    at Object.exports.renderFile (/sass-issue/node_modules/pug/lib/index.js:410:38)
    at Object.exports.renderFile (/sass-issue/node_modules/pug/lib/index.js:400:21)
    at View.exports.__express [as engine] (/sass-issue/node_modules/pug/lib/index.js:447:11)
    at View.render (/sass-issue/node_modules/express/lib/view.js:126:8)
    at tryRender (/sass-issue/node_modules/express/lib/application.js:639:10)
    at EventEmitter.render (/sass-issue/node_modules/express/lib/application.js:591:3)
    at ServerResponse.render (/sass-issue/node_modules/express/lib/response.js:960:7)
    at app.get (/sass-issue/app/index.js:10:32)
    at Layer.handle [as handle_request] (/sass-issue/node_modules/express/lib/router/layer.js:95:5)
    at next (/sass-issue/node_modules/express/lib/router/route.js:131:13)
    at Route.dispatch (/sass-issue/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/sass-issue/node_modules/express/lib/router/layer.js:95:5)
    at /sass-issue/node_modules/express/lib/router/index.js:277:22
    at Function.process_params (/sass-issue/node_modules/express/lib/router/index.js:330:12)
    at next (/sass-issue/node_modules/express/lib/router/index.js:271:10)
    at middleware (/sass-issue/node_modules/connect-assets/index.js:46:7)
    at Layer.handle [as handle_request] (/sass-issue/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/sass-issue/node_modules/express/lib/router/index.js:312:13)
    at /sass-issue/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/sass-issue/node_modules/express/lib/router/index.js:330:12)
    at next (/sass-issue/node_modules/express/lib/router/index.js:271:10)
    at expressInit (/sass-issue/node_modules/express/lib/middleware/init.js:33:5)
    at Layer.handle [as handle_request] (/sass-issue/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/sass-issue/node_modules/express/lib/router/index.js:312:13)
    at /sass-issue/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/sass-issue/node_modules/express/lib/router/index.js:330:12)
    at next (/sass-issue/node_modules/express/lib/router/index.js:271:10)
    at query (/sass-issue/node_modules/express/lib/middleware/query.js:44:5)
    at Layer.handle [as handle_request] (/sass-issue/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/sass-issue/node_modules/express/lib/router/index.js:312:13)
    at /sass-issue/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/sass-issue/node_modules/express/lib/router/index.js:330:12)
    at next (/sass-issue/node_modules/express/lib/router/index.js:271:10)
    at Function.handle (/sass-issue/node_modules/express/lib/router/index.js:176:3)
    at EventEmitter.handle (/sass-issue/node_modules/express/lib/application.js:173:10)
    at Server.app (/sass-issue/node_modules/express/lib/express.js:38:9)
    at emitTwo (events.js:106:13)
    at Server.emit (events.js:191:7)
    at HTTPParser.parserOnIncoming [as onIncoming] (_http_server.js:546:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:99:23)

$ node -v
v6.9.2

Demo app that reproduces the error is here: http://github.com/deployable/sass-issue (probably needs node6+)

git clone http://github.com/deployable/sass-issue.git
npm install
npm test
npm start

connect-assets can serve the .css and .css.map files ok from the web app

curl http://localhost:3001/assets/bootstrap.css
curl http://localhost:3001/assets/bootstrap.css.map

The node-sass binary can generate the css and map files fine

$ cd app/assets/css/
$ ../../../node_modules/.bin/node-sass -o . --source-map true bootstrap.sass 
Rendering Complete, saving .css file...
Wrote CSS to /sass-issue/app/assets/css/bootstrap.css
Wrote Source Map to /sass-issue/app/assets/css/bootstrap.css.map