Node assert.throws not catching exception

Given this code:

var assert = require('assert');

function boom(){
    throw new Error('BOOM');
}

assert.throws( boom(), Error );

I get this output, with node 0.4.9:

node.js:134
        throw e; // process.nextTick error, or 'error' event on first tick
        ^
Error: BOOM
    at boom ([EDITED]/assert.throws.test.js:4:9)
    at Object.<anonymous> ([EDITED]/assert.throws.test.js:7:17)
    at Module._compile (module.js:402:26)
    at Object..js (module.js:408:10)
    at Module.load (module.js:334:31)
    at Function._load (module.js:293:12)
    at Array.<anonymous> (module.js:421:10)
    at EventEmitter._tickCallback (node.js:126:26)

This, to me, implies that an uncaught exception has occurred, as opposed to a reported, caught exception. Looking in the docs, I notice that the examples look more like this:

var assert = require('assert');

function boom(){
    throw new Error('BOOM');
}

assert.throws( boom, Error );

But how do you test if it throws an exception given a certain input? For example:

var assert = require('assert');

function boom(blowup){
    if(blowup)
        throw new Error('BOOM');
}

assert.throws( boom, Error );

This will fail. What am I doing wrong, or what secret does everybody know but me?