Error reflashing Nexus S to stock Jelly Bean

I've got a Nexus S (D720) running CM 10.1.3, and I'd like to flash it back over to Google's latest factory image (sojus-jro03r). When I try updating, I get the following: sending 'bootloader' (1536 KB)... OKAY [ 0.195s] writing 'bootloader'... OKAY [ 0.347s] finished. total time: 0.542s rebooting into bootloader... OKAY [ 0.001s] finished. total time: 0.001s sending 'radio' (15232 KB)... OKAY [ 2.037s] writing 'radio'... OKAY [ 15.314s] finished. total time: 17.350s rebooting into bootloader... OKAY [ 0.001s] finished. total time: 0.001s archive does not contain 'boot.sig' archive does not contain 'recovery.sig' archive does not contain 'system.sig' -------------------------------------------- Bootloader Version...: D720SPRKE1 Baseband Version.....: D720SPRXXX Serial Number........: 3730DA5FBAF300EC -------------------------------------------- checking product... OKAY [ 0.001s] checking version-bootloader... OKAY [ 0.001s] checking version-baseband... FAILED Device version-baseband is 'D720SPRXXX'. Update requires 'D720SPRKC9' or 'D720SPRKD8' or 'D720SPRKE5' or 'D720SPRKH1'. finished. total time: 0.006s Searching "D720SPRXXX" on Google gets exactly zero hits. How did my baseband version get like that, and how do I change it to make Google's update happy? It's worth noting that the device *still boots* into CyanogenMod without issues, and can be reflashed with a *new* version of CyanogenMod; I just can't install Google's factory image.