Best response from calculator for uncomputable expression

What should be the best response from a calculator or computer algebra system when it can't evaluate given expression ? Some examples:

LibreCalc         : =?÷?  → Err:501
Ubuntu Calculator : ?÷?   → Malformed expression
Wolfram Alpha     : ?÷?   → Wolfram|Alpha doesn't understand your query
Maxima            : maxima --very-quiet -r '?÷?;' → ?÷?

Now my personal impressions about these responses:

Err:501

Some cryptic error code which doesn't say anything about issue at hand. You must then read manuals to understand what's going on. And from version-to-version this error code can change.

Malformed expression

This tries to argue that my given expression is somewhat BAD. But you can't be sure about it. It may be bad OR simply calculator engine don't have an internal knowledge how to compute that expression. So this response is too categorical.

Wolfram|Alpha doesn't understand your query

This is much, much better. Shows that computation engine can't parse requested expression. Of course if I would get such answer from a human, I would ask "What you don't understand exactly ?" So still not 100% right.

?÷?

I choose this as a winner. This program tries to be mathematically correct in each and every way. Every man on the world knows that expression is equal to itself : x=x. So if you can't evaluate given expression - at least show to the user that you know this principle. It respects my feeling
"Do something !". And it does (at least what it can)

What do you think ?