Expression Editor with various column types

We use the expression editor extensively and love it.

It works great for almost all of our numeric columns. (Int32, Int64, double, datetime) however, for Nullable Decimal types, we can not use Functions.

I can do columnName * columnName and the Result Preview works, and the Expression is valid.

however, if I do Sqrt(columnName) or any other function the Result Preview is blank, and the Expression property is null?

Any suggestions?