§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.
§2. Conditions attached to a vote before the election are to be considered as not having been added.
Source: Code of Canon Law (Vatican.va)