You're right, I mischaracterized it as a grammar issue. It was a usage issue. Maybe you can substitute more general synonyms - which are deeply buried in any dictionary definition I can find, if there at all - and make it sound reasonable, but incredulous specifically refers to a human state. Probably the best one-word synonym would be skeptical.
Why not just admit it wasn't what you meant to type? You always find it necessary to dig yourself into deeper holes when you make a mistake or say something silly off the cuff.