Internal objectdb error when open a database or open an entity manager

we get following exception when we open a database or open an entity manager

Exception in thread "ODB IndexActivation" [ObjectDB 2.6.6_06] Unexpected exception (Error 990)
  Generated by Java HotSpot(TM) 64-Bit Server VM 1.8.0_51 (on Windows 7 6.1).
Please report this error on http://www.objectdb.com/database/issue/new
com.objectdb.o.InternalException: null
com.objectdb.o.InternalException
at com.objectdb.o.InternalException.f(InternalException.java:236)
at com.objectdb.o.IXM.J(IXM.java:584)
at com.objectdb.o.IXM.I(IXM.java:558)
at com.objectdb.o.IIS.j(IIS.java:67)
at com.objectdb.o.IXS.i(IXS.java:92)
at com.objectdb.o.IIS.p(IIS.java:55)
at com.objectdb.o.IXM.Vf(IXM.java:439)
at com.objectdb.o.PBI.C(PBI.java:146)
at com.objectdb.o.PBI.q(PBI.java:115)
at com.objectdb.o.MBI.VC(MBI.java:83)
at com.objectdb.o.MST.Vn(MST.java:783)
at com.objectdb.o.IXM.D(IXM.java:364)
at com.objectdb.o.IXM.run(IXM.java:339)
at java.lang.Thread.run(Thread.java:745)

Thank you for this report. The stack trace indicates an issue in the new index activation that was added in version 2.6.6 (probably when there are classes with no persistent fields in the hierarchy).

Build 2.6.6_07 should fix this exception.