Allah has exempted her of His rights...

Allah has exempted her of His rights: why then must she fast because of what she has placed from Allah upon herself?’ I then asked him (the Imam), ‘What will happen if she did not do it at all?’ He (the Imam) said, ’If so, I fear for her to experience in the matter for which she had vowed to see what she may dislike.’”

وثق في قوة الصحيح - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (16/354))