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

1. A agrees, in writing, to sell a horse of B for Rs. 1,000 or Rs. 1,500.
   Evidence cannot be given to show which price was to be given.
2. A deed contains blanks. Evidence cannot be given of facts which would show how they were meant to be filled.