soapui 5.x script assertion not implemented for json error

since upgrading to the latest soapUI pro version (5.1.2) I have noticed that I cannot right click on a node in the outline response mode and use the 'Add Assertion -> for Existence with Script' feature. This feature works in the older versions of soapUI. I need this because I need to get the correct namespace setting for the node I want to assert. Does anyone know if this is a limitation of the newer version or soapUI ? or do I need to tweak the configuration somewhere?