Are oozes immune to critical failures?

In a previous session, one of my players used Sudden Bolt against a Living Sap. Living Sap’s have immunity to critical hits. They rolled low on their Reflex Save, and got 11 less than the spellcaster’s DC. I ruled critical failure and gave Sudden Bolt double damage. One of my players had assumed that their immunity to critical hits would make them immune to critical failures (they didn’t fight too hard since it was a fairly beneficial ruling 🙂 ). Was I right to rule that way? (I’ve included my reasoning as an answer on the off chance that I’m right).