Mongoose virtual set function not called when value is object

I have a mongoose schema with a virtual of mixed type. For example:

var mongoose = require('mongoose') // version 3.3.1

var FooSchema = new mongoose.Schema( { x: Number } );

FooSchema.virtual('v').set( function(value){ 
  console.log("SETTING", value); 
});

var Foo = mongoose.model('Foo', FooSchema);

new Foo( { v:1 } );
new Foo( { v:[] } );
new Foo( { v:{} } );

When I run this code I get:

SETTING 1
SETTING []

As you'll notice it never shows "SETTING {}", any reason why this doesn't work?

Reference to issue on Github