Knock knock. Who's there? Syntax error.

Syntax error who? Please try again with a valid input.

Knock knock. Who's there? Donut.

Donut who? Donut even think about glazing over this punchline; it's sure to be sweet!

Knock knock. Who's there? Citation needed.

Citation needed who? This joke requires a source to be funny.

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not really sure, because the concept of identity and self is fleeting and meaningless in an infinite universe.