Ai wa Muteki ~Hatachi no Yoru no Chikai~ (愛は無敵〜二十歳の夜の誓い〜; Love Is Invincible ~An Oath I Made At Night When I Turned 20~) is Sheki-Dol's fourth single. It is also their first major debut single, and last single. This is the only release featuring third-generation member Arai Saki.
"Ai wa Muteki ~Hatachi no Yoru no Chikai~" was meant to be the group's major debut single, but the unit disbanded soon after release when it was announced that Suenaga Mami needed medical care. It was later revealed that she had had a break-down due to stress.