(CNN) – In 1958, artist Yves Klein famously opened an exhibition called "The Void," which saw him place a large cabinet in an otherwise empty room. Thousands of paying visitors turned up at a Paris gallery to catch a glimpse of nothing at all.
Following the show's success, the French artist then took the idea one step further – by giving collectors the chance to purchase a series of non-existent and entirely conceptual spaces in exchange for a weight of pure gold.
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A handful of buyers took him up on the offer. And now, almost 60 years after Klein's death, one of the receipts he wrote to prove ownership of his invisible artworks is up for sale, with auction house Sotheby's estimating that it could fetch up to 500,000 euros ($551,000).