oracle ELSE case runs regardless

Question

I have a case statement (myCol is a NUMBER):

select case when myCol < 0 then 0 else round(POWER(10,AVG(LOG(10,myCol))),2) end myCol from myTable where id = 123 group by myCol;

But Oracle seems to run the else clause regardless, and the POWER or LOG functions throw ORA-01428: argument '-2.75' is out of range when myCol is negative (-2.75). I would think the else clause would be ignored if myCol < 0 but maybe Oracle is compiling the entire statement before running?

I also tried using decode(sign(myCol)... but that also fails.

How can I handle this?