Is a vow contingent upon a condition unlawful, and is expiation obligatory for one who makes a vow contingent upon a condition?

A vow conditioned on an event, such as the birth of a child, is considered disliked (makruh) by most scholars. Some even deem it forbidden (haram) if it is accompanied by a corrupt belief. Fulfillment of such a vow becomes obligatory if the condition is met; otherwise, nothing is binding.