Ruby forcing garbage collection not working as expected

In the following code:

class ExampleClass
  def initialize
    ObjectSpace.define_finalizer(self, proc{puts 'dead'})
  end
end

ex = ExampleClass.new
ex = nil

GC.start

while true
  # the while loop is to prevent the program from terminate
  # if the program does terminate, the finalizer gets called in the end like expected
end

The finalizer here is never called and there is no output. I would have expected the garbage collector to collect ex since it has been dereferenced. Why is GC.start not forcing ex to be collected and causing the finalizer to be called immediately?