Cannot find module requirejs when installed globally

I have installed requirejs globally, but node says it can't find it.

# node tools/mapconv.js

module.js:340
    throw err;
          ^
Error: Cannot find module 'requirejs'
    ...

# cat mapconv.js
var requirejs = require('requirejs');
...

The module is definitely installed:

# npm ls -g | grep requirejs
├── requirejs@2.1.6

I have cleared the NPM cache, but no change. Has anyone else run into this?

Thanks.