Merging patient records works, but shows a stack trace

I was trying to reproduce RA-663 on the demo server and I got a stack trace while trying to merge patients:

java.lang.NullPointerException
	at org.openmrs.module.coreapps.page.controller.datamanagement.MergePatientsPageController.post(MergePatientsPageController.java:125)
...

Page URL was http://demo.openmrs.org/openmrs/coreapps/datamanagement/mergePatients.page?app=coreapps.mergePatients&isUnknownPatient=false&patient1=42&patient2=43

Also, at this point the patients actually are merged, notwithstanding the stack trace.