Logical test with multiple "true" conditions

I know there is a multitude of questions on this function/topic, but I couldn't find this specific one.

I am digging through data looking for logical errors.

Basically, I am trying to ask:

$A  $B
3   R
5   Y
5   Y
5   R
6   R
5   Z

If column A equals "5", then column B should equal Y or Z. If not, give "ERROR".

All other values in Column A can be ignored.

The result should be:

$A  $B  $C
3   R   
5   Y   
5   Y   
5   R   ERROR
6   R   
5   Z