Use SWITCH in PowerQuery Transform steps

I would like to use a column with a SWITCH formula in PowerQuery to identify bad values and replace them. In this example I have only one case, "#N/A", and the word "CHECK" to flag these for now. The Custom Column box gives me a checkmark of approval that No syntax errors have been detected. When I hit enter, the formula bar gives me an Expression Error. How can I use  a SWITCH here?

PS I want to do this in the PowerQuery transformations, NOT after by adding a column, because there are additional tables derived from this one and I want the column included in them (without manually duplicating it for each derived table).