When the language used in a document is, on its face, ambiguous or defective, evidence may not be given of facts which would show its meaning or supply its defects.
     Illustrations
     (a) A agrees, in writing, to sell a horse to B for Rs. 1,000 or Rs. 1,500. Evidence cannot be given to show which price was to be given.
     (b) A deed contains blanks. Evidence cannot be given of facts which would show how they were meant to be filled.