Failed to load custom visualization

Hi! I have followed the steps for creating a customized visualization (see http://docs.splunk.com/Documentation/Splunk/6.4.0/AdvancedDev/CustomVizTutorial), but when I am testing the visualization in Splunk Web I get the following error: "Failed to load source for Radial Meter visualization." The problem is probably because running "npm run build" (see step 1 beneath "Rebuild the visualization") does not generate visualization.js. Does anybody have any thoughts on why this is not working? Julie