Gjs doesn't execute Vala constructors

When constructing a new instance of a class Gjs doesn't correctly call constructors for libraries written in Vala. I've included a simple example below.

namespace GjsConstructorTest {
    public class TestClass : GLib.Object
    {
        private int field;

        public TestClass () {
            this.field = 5;
        }

        public int getField () {
            return this.field;
        }
    }
}

This Gjs code should print 5 but it instead prints 0 indicating the constructor was never called.

const TestClass = imports.gi.GjsConstructorTest.TestClass
let test = new TestClass()
print(test.getField())