Why not just use VSTP config option, set protocols vstp vlan "X" instead? VSTP was designed by Juniper to allow interoperability with non-standard PVSTP/PVSTP+ from Cisco. For + you might need both vstp and rstp configured at same time.
Not sure which model EX you are trying this with (always good idea to include both model and SW version), but below is link to newer ELS model discussion of STP Options:
I am also attaching an old (but good) document regarding 3rd-party interoperability testing between Juniper and Cisco.
Good luck, hope this helps.