TestFieldMaskingSpanQuery suite failure: 3 segments exist after a forceMerge(1)

Checking out Revision 7b93fc46b1e284494a500f4df541269bdfa67dcf (refs/remotes/origin/master)
[...]
   [junit4] Suite: org.apache.lucene.search.spans.TestFieldMaskingSpanQuery
   [junit4]   2> NOTE: test params are: codec=Asserting(Lucene60): {gender=FSTOrd50, last=PostingsFormat(name=LuceneVarGapDocFreqInterval), id=BlockTreeOrds(blocksize=128), first=Lucene50(blocksize=128)}, docValues:{}, maxPointsInLeafNode=734, maxMBSortInHeap=5.661209237449429, sim=RandomSimilarity(queryNorm=true,coord=no): {gender=DFR I(n)3(800.0), last=DFR I(n)L1, id=DFR I(n)L1, first=IB SPL-L2}, locale=fr, timezone=Etc/GMT+11
   [junit4]   2> NOTE: Linux 4.1.0-custom2-amd64 amd64/Oracle Corporation 1.8.0_45 (64-bit)/cpus=16,threads=1,free=213068040,total=315621376
   [junit4]   2> NOTE: All tests run in this JVM: [TestInfoStream, TestBooleanOr, TestBooleanCoord, TestNRTThreads, TestMultiset, TestTopDocsCollector, TestIndexWriterMaxDocs, TestNIOFSDirectory, TestIntArrayDocIdSet, TestDocBoost, TestPositiveScoresOnlyCollector, TestIntBlockPool, TestLegacyNumericUtils, TestPersistentSnapshotDeletionPolicy, TestIndexWriterOnJRECrash, TestIndexWriterOutOfFileDescriptors, TestTimSorter, TestIndexWriterUnicode, TestSearcherManager, TestUTF32ToUTF8, TestLucene50CompoundFormat, TestFieldCacheRewriteMethod, TestVirtualMethod, TestSegmentInfos, TestFuzzyQuery, TestScoreCachingWrappingScorer, TestSpansEnum, TestMultiPhraseQuery, TestTragicIndexWriterDeadlock, TestIndexWriterNRTIsCurrent, TestPointQueries, TestMultiTermConstantScore, TestComplexExplanationsOfNonMatches, TestDocValues, TestSpanCollection, TestTopFieldCollector, TestIndexSearcher, Test2BPostings, TestBM25Similarity, TestRollback, TestSimpleAttributeImpl, TestDocumentsWriterStallControl, TestSearch, TestBinaryDocValuesUpdates, TestIsCurrent, TestTermVectorsReader, TestUniqueTermCount, TestRollingBuffer, TestSleepingLockWrapper, TestSortedSetSelector, TestEarlyTermination, TestConjunctions, TestArrayUtil, TestSpanNearQuery, Test2BSortedDocValuesOrds, TestNumericTokenStream, TestSparseFixedBitSet, TestNotDocIdSet, TestDirectory, TestBytesRefAttImpl, TestSortRescorer, TestDeterminism, TestDocumentWriter, TestParallelLeafReader, TestIndexWriterLockRelease, TestTermdocPerf, TestSloppyPhraseQuery, TestSpanFirstQuery, TestApproximationSearchEquivalence, TestConsistentFieldNumbers, TestRateLimiter, TestSloppyMath, TestHighCompressionMode, TestSubScorerFreqs, TestBKD, TestRegExp, TestOmitPositions, TestMergePolicyWrapper, TestPerFieldDocValuesFormat, TestTotalHitCountCollector, TestAddIndexes, TestSegmentMerger, TestForceMergeForever, TestLSBRadixSorter, TestNRTCachingDirectory, TestLucene50StoredFieldsFormatHighCompression, TestIndexWriterReader, TestSimilarity, TestRoaringDocIdSet, TestFieldMaskingSpanQuery]
   [junit4]   2> NOTE: download the large Jenkins line-docs file by running 'ant get-jenkins-line-docs' in the lucene directory.
   [junit4]   2> NOTE: reproduce with: ant test  -Dtestcase=TestFieldMaskingSpanQuery -Dtests.seed=EEB9C1523889BD0F -Dtests.slow=true -Dtests.linedocsfile=/home/jenkins/lucene-data/enwiki.random.lines.txt -Dtests.locale=fr -Dtests.timezone=Etc/GMT+11 -Dtests.asserts=true -Dtests.file.encoding=UTF-8
   [junit4] ERROR   0.00s J1 | TestFieldMaskingSpanQuery (suite) <<<
   [junit4]    > Throwable #1: java.lang.IllegalArgumentException: StandardDirectoryReader(segments_1:8 _0(7.0.0):c2 _1(7.0.0):c2 _2(7.0.0):c1) has 3 segments instead of exactly one
   [junit4]    > 	at __randomizedtesting.SeedInfo.seed([EEB9C1523889BD0F]:0)
   [junit4]    > 	at org.apache.lucene.util.LuceneTestCase.getOnlyLeafReader(LuceneTestCase.java:789)
   [junit4]    > 	at org.apache.lucene.search.spans.TestFieldMaskingSpanQuery.beforeClass(TestFieldMaskingSpanQuery.java:121)
   [junit4]    > 	at java.lang.Thread.run(Thread.java:745)
   [junit4] Completed [171/415 (1!)] on J1 in 0.02s, 0 tests, 1 error <<< FAILURES!