Nodeunit with grunt can't find module 'tap'

So, armed with this tutorial, I decided to add some unit testing to my node.js app. My gruntfile seems to be OK, and when I type grunt nodeunit my only test runs just fine, but after that, it crashes with error Fatal error: Cannot find module 'tap':

$> grunt nodeunit
Running "nodeunit:all" (nodeunit) task
Testing db.test.js
.OK
Fatal error: Cannot find module 'tap'

I didn't know anything about this module, but after googliing it up, it seemed that it's something nodeunit would require. And indeed, there it is: $MY_NODE_APP/node_modules/nodeunit/node_modules/tap exists, and when I launch node in $MY_NODE_APP/node_modules/nodeunit and type require('tap') in interactive console, I get some object with a lot of stuff that gives me an impression that it works as it should.

So, the obvious question: why do I get this error, and how can I fix it?

Gruntfile:

module.exports = function(grunt) {

    grunt.initConfig({
        nodeunit: {
            all: ['./**/*.test.js']
        }
    });

    grunt.loadNpmTasks('grunt-contrib-nodeunit');
};

Update: Installed tap to my project, but it didn't help too.

I solved the issue (well, this issue — it wasn't the last one) by installing the tap module manually.