Questions on Constructivism

How is it that constructivists know we can not know objective reality? Wouldn't one have to know objective reality to objectively know that we cannot know objective reality? (Wow, that's a lot of "objective" words). It seems to me that the underlying statement is, "We cannot know objective reality, except that we know we cannot know objective reality." (apologies if I am setting up a "straw-man"...I'm posing the statement to better clarify my question)