NullPointerException on SortBuffer.removeFirst()

While running simple select statement we got the following error:

ERROR XJ001: Java exception: ': java.lang.NullPointerException'.

The query we ran was from the following form:

select this_.id as y0_, ...<more_attributes> from <table> this_ where this_.<some_attr>=<some_value> order by y0_ asc fetch first 1000 rows only

Full Log and Stack Trace:
----------------------------------------------------------------
Tue Nov 15 21:33:17 IST 2016:
Booting Derby version The Apache Software Foundation - Apache Derby - 10.13.1.1 - (1765088): instance a816c00e-0158-697a-aef3-0000427a785d
on database directory /local/ntr/vlnx577.159043/local_server/vmgr.db with class loader sun.misc.Launcher$AppClassLoader@312b1dae
Loaded from file:/view/niroren_ngv_dev_vm_fst_upgrade_derby_pb/vobs/dvproject/.m2/repository/org/apache/derby/derby/10.13.1.1/derby-10.13.1.1.jar
java.vendor=Oracle Corporation
java.runtime.version=1.8.0_66-b17
user.dir=/local/ntr/vlnx577.159043
os.name=Linux
os.arch=amd64
os.version=2.6.32-431.11.2.el6.x86_64
derby.system.home=/local/ntr/vlnx577.159043/local_server
Database Class Loader started - derby.database.classpath=''
----------------------------------------------------------------
Tue Nov 15 21:33:35 IST 2016:
Shutting down instance a816c00e-0158-697a-aef3-0000427a785d on database directory /local/ntr/vlnx577.159043/local_server/vmgr.db with class loader sun.misc.Launcher$AppClassLoader@312b1dae
----------------------------------------------------------------
Tue Nov 15 21:33:39 IST 2016:
Booting Derby version The Apache Software Foundation - Apache Derby - 10.13.1.1 - (1765088): instance 0a0c9f2c-0158-697a-aef3-0000427a785d
on database directory /local/ntr/vlnx577.159043/local_server/vmgr.db with class loader sun.misc.Launcher$AppClassLoader@312b1dae
Loaded from file:/view/niroren_ngv_dev_vm_fst_upgrade_derby_pb/vobs/dvproject/.m2/repository/org/apache/derby/derby/10.13.1.1/derby-10.13.1.1.jar
java.vendor=Oracle Corporation
java.runtime.version=1.8.0_66-b17
user.dir=/local/ntr/vlnx577.159043
os.name=Linux
os.arch=amd64
os.version=2.6.32-431.11.2.el6.x86_64
derby.system.home=/local/ntr/vlnx577.159043/local_server
Database Class Loader started - derby.database.classpath=''
Tue Nov 15 22:09:07 IST 2016 Thread[pool-11-thread-9,5,main] (XID = 14429), (SESSIONID = 3), (DATABASE = vmgr.db), (DRDAID = null), Cleanup action starting
Tue Nov 15 22:09:07 IST 2016 Thread[pool-11-thread-9,5,main] (XID = 14429), (SESSIONID = 3), (DATABASE = vmgr.db), (DRDAID = null), Failed Statement is: select this_.id as y0_, ... from .... this_ where ... order by y0_ asc fetch first 1000 rows only with 1 parameters begin parameter #1: .... :end parameter
java.lang.NullPointerException
at org.apache.derby.impl.store.access.sort.SortBuffer.removeFirst(Unknown Source)
at org.apache.derby.impl.store.access.sort.SortBufferScan.next(Unknown Source)
at org.apache.derby.impl.sql.execute.SortResultSet.getRowFromSorter(Unknown Source)
at org.apache.derby.impl.sql.execute.SortResultSet.getNextRowFromRS(Unknown Source)
at org.apache.derby.impl.sql.execute.SortResultSet.getNextRowCore(Unknown Source)
at org.apache.derby.impl.sql.execute.RowCountResultSet.getNextRowCore(Unknown Source)
at org.apache.derby.impl.sql.execute.BasicNoPutResultSetImpl.getNextRow(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedResultSet.movePosition(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedResultSet.next(Unknown Source)
at com.mchange.v2.c3p0.impl.NewProxyResultSet.next(NewProxyResultSet.java:684)
at org.hibernate.loader.Loader.processResultSet(Loader.java:975)
at org.hibernate.loader.Loader.doQuery(Loader.java:937)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:343)
at org.hibernate.loader.Loader.doList(Loader.java:2609)
at org.hibernate.loader.Loader.doList(Loader.java:2592)
at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2424)
at org.hibernate.loader.Loader.list(Loader.java:2419)
at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:109)
at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1855)
at org.hibernate.internal.CriteriaImpl.list(CriteriaImpl.java:365)
at ....... <application_code>
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:333)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:213)
at com.sun.proxy.$Proxy73.find(Unknown Source)
at ...... <application_code>
at javax.swing.SwingWorker$1.call(SwingWorker.java:295)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at javax.swing.SwingWorker.run(SwingWorker.java:334)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Cleanup action completed

This failure, however, is not consistent and occures during our nightly regression, when the same database query is executed multiple times.

Regards,
Nir