Average value display using Expression Tag

I have a summary dashboard. It has to display the average value of individual plant sites.
I have assigned expression tag with average. However it shows as null value if one of the plant site is down. How to get the average value by only considering the plants which is up.
This is the view which shows null value
I have assigned expression tag to the label as shown below

Here is the expression tag assigned

the expression in the tag is as follows

Please help me out with this

What's the context of the error? (go into the diagnostics of the expression tag and check what the actual error description is, or mouse over the error text in the binding preview)