Regression in 3.5.5: Cascade on merge for Sets fails with transient entities

Something happened from 3.5.4 to 3.5.5. Here is regression issue we're experiencing after upgrade:

We have entities ResearchClientFile and ResearchVendorRecord.

RCF maps RVR:

@Sort(type = SortType.NATURAL)
@OneToMany(mappedBy = "file", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
public SortedSet<ResearchVendorRecord> getRecords() {
return records;
}

RVR maps RCF:

@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "research_client_file_id")
public ResearchClientFile getFile() {
return file;
}

The following unit test:

ResearchClientFile rcf = RandomUtils.randomBean(ResearchClientFile.class);
rcf.addRecord(RandomUtils.randomBean(ResearchVendorRecord.class));
rcf = session.merge(rcf);

produces the following exception in 3.5.5 but not in 3.5.4.:

java.lang.NullPointerException: null entities are not supported by org.hibernate.event.def.EventCache
at org.hibernate.event.def.EventCache.containsKey(EventCache.java:80)
at org.hibernate.event.def.DefaultMergeEventListener.mergeTransientEntity(DefaultMergeEventListener.java:361)
at org.hibernate.event.def.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:303)
at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:258)
at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:869)

equals, hashCode and compareTo are completely consistent in RVR.

The issue seems to affects only Set and SortedSets and on merge only. Lists (with bag semantics) do not appear to be affected on merge.