The `supportRelativeURL` function always assumes mappings are relative, even when they may be an absolute URI. Given: - Build 0.3.2 of source-map-support - a **file** of `c:\dir\a.min.js` - a **url** of `file:///C:/dir/a.js` The result is `C:\dir\file:\C:\dir\a.js` when it should be `C:\dir\a.js`. The issue stems from the fact node's `path.resolve` function does not interpret file URIs.