Canon 172

Can. 172 §1. To be valid, a vote must be:
1° free; therefore the vote of a person who has been coerced directly or indirectly by grave fear or malice to vote for a certain person or different persons separately is invalid;
2° secret, certain, absolute, determined.

Can. 172 §2. Conditions attached to a vote before the election are to be considered as not having been added.