Grails Upgrade 2.4.4 to 2.5.2 Error running unit t

Question

I'm upgrading a Grails app from 2.4.4 to 2.5.2 and when I try and run test-app, I'm getting the following:

Error |
Error running unit tests: null
java.lang.AbstractMethodError
    at org.spockframework.runtime.GlobalExtensionRegistry.startGlobalExtensions(GlobalExtensionRegistry.java:105)
    at org.spockframework.runtime.RunContext.start(RunContext.java:63)
    at org.spockframework.runtime.RunContext.get(RunContext.java:165)

Google hasn't been much help. I've done a clean-all, deleted the .grails/2.5.2 folder but just can't track down the root cause.