Imaginary Tubes

Randomly, instead of generating an image, it instead generates the following error:

TypeError: Cannot read property 'x' of undefined
   at /app/metro.js:424:29
   at Array.map (native)
   at rasterizeLines (/app/metro.js:409:31)
   at generateSegments (/app/metro.js:473:27)
   at tubeLine (/app/metro.js:585:34)
   at Object.tubeLines (/app/metro.js:600:7)
   at tubeImage (/app/server.js:49:23)
   at /app/server.js:143:37
   at Layer.handle [as handle_request] (/app/node_modules/express/lib/router/layer.js:95:5)
   at next (/app/node_modules/express/lib/router/route.js:131:13)
   at Route.dispatch (/app/node_modules/express/lib/router/route.js:112:3)
   at Layer.handle [as handle_request] (/app/node_modules/express/lib/router/layer.js:95:5)
   at /app/node_modules/express/lib/router/index.js:277:22
   at Function.process_params (/app/node_modules/express/lib/router/index.js:330:12)
   at next (/app/node_modules/express/lib/router/index.js:271:10)
   at SendStream.error (/app/node_modules/serve-static/index.js:121:7)

This looks like a fun project! Seems like the turns going from north -> east don't line up with the circles all the way. I really like how intricate some of the maps get.