law: Indian Contract Act, 1872 
  20. Agreement void where both parties
  
  are under mistake as to matter of fact 
  Where both the parties to an agreement are under a mistake as to
  
  a matter of fact essential to the agreement, the agreement, the agreement is
  
  void.
  Explanation:  An erroneous opinion as to the value of the
  
  things which forms the subject-matter of the agreement, is not be deemed a
  
  mistake as to a matter of fact.
   Illustrations 
  (a) A agrees to sell to B a specific cargo of goods supposed to
  
  be on its way from England to Bombay. It turns out that, before the day of the
  
  bargain in the ship conveying the cargo had been cast away and the goods lost.
  
  Neither party was aware of these facts. The agreement is void.
  (b) A agrees to buy from B a certain horse. It turns out that
  
  the horse was dead at the time of bargain, though neither party was aware of
  
  the fact. The agreement is void.
  (c) A, being entitled to an estate of the life of B, agrees to
  
  sell it to C, B was dead at the time of the agreement, but both parties were ignorant
  
  of the fact. The agreement is void.