I'm experiencing similar behavior. When I use a "Web Service Data Source" and check if a property of the result is null, it always returns false. I assume because the underlying object is an empty JValue. When I use the IsNull function I get the same error as Bharat has reported above.
Based on the following JSON, when the "prop2" is null, I'd like to hide a panel that contains a text box displaying the "innerVal".
I tried that by assigning a conditional formatting rule to the panel that hides the panel when
Expression: = Fields.prop2
But that didn't work.
How would I display/hide some content of the report in case the value is not provided in the JSON result?