As I cannot reproduce the same error in the Mendix-version I use (8.6.3), some basic questions:
Your expression should be a valid one and works in a local test of mine. Can you please provide some more detail about where the conditional visibility is being applied? It will need to be inside a data view or list view where the $currentObject is available. Is it possible this widget is outside the bounds of the data view where desired $currentObject is available?
If not, can you provide a screenshot of the page you are trying to manipulate?
i know it’s a year ago but still maybe this answer helps you or someone else.
I ran into the same problem and I figured that this error always came up when it’s checking the condition on an ‘empty’ value. So in your example when your $currentObject/value is empty instead of a decimal value.
I solved this by adding a condition that first checks if it’s empty.