Sytem Image Utility can't find Installer App
I'm trying to create a new NetInstall image to update one of our labs but System Image Utility doesn't offer any installation sources in the 'Choose a Source' screen. I have the latest version of 'Install macOS Sierra' on disk and the SIU preferences point to the correct folder where that app is stored (ironically the file dialog to select the location of the macOS installers even shows the installer apps, greyed out).
System Image Utility: Version 10.12 (807)
OSX Server: Version 5.3.1 (16S4128)
OS: macOS Sierra - Version 10.12.6
Any idea what went wrong?
OS X El Capitan (10.11.6)