diff --git a/lucene/test-framework/src/java/org/apache/lucene/index/BasePostingsFormatTestCase.java b/lucene/test-framework/src/java/org/apache/lucene/index/BasePostingsFormatTestCase.java
index 2ee380e3478..5c2bab39587 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/index/BasePostingsFormatTestCase.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/index/BasePostingsFormatTestCase.java
@@ -34,6 +34,7 @@ import java.util.Set;
 import java.util.SortedMap;
 import java.util.TreeMap;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.lucene.analysis.MockAnalyzer;
 import org.apache.lucene.codecs.Codec;
@@ -1381,7 +1382,12 @@ public abstract class BasePostingsFormatTestCase extends LuceneTestCase {
     Directory dir = newDirectory();
     IndexWriterConfig iwc = newIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random()));
 
-    final Map<String,TermFreqs> termFreqs = new HashMap<String,TermFreqs>();
+    // Must be concurrent because thread(s) can be merging
+    // while up to one thread flushes, and each of those
+    // threads iterates over the map while the flushing
+    // thread might be adding to it:
+    final Map<String,TermFreqs> termFreqs = new ConcurrentHashMap<String,TermFreqs>();
+
     final AtomicLong sumDocFreq = new AtomicLong();
     final AtomicLong sumTotalTermFreq = new AtomicLong();
 
@@ -1398,6 +1404,8 @@ public abstract class BasePostingsFormatTestCase extends LuceneTestCase {
           }
           final PostingsFormat defaultPostingsFormat = p;
 
+          final Thread mainThread = Thread.currentThread();
+
           if (field.equals("body")) {
 
             // A PF that counts up some stats and then in
@@ -1420,6 +1428,15 @@ public abstract class BasePostingsFormatTestCase extends LuceneTestCase {
 
                     boolean isMerge = state.context.context == IOContext.Context.MERGE;
 
+                    // We only use one thread for flushing
+                    // in this test:
+                    assert isMerge || Thread.currentThread() == mainThread;
+
+                    // We iterate the provided TermsEnum
+                    // twice, so we excercise this new freedom
+                    // with the inverted API; if
+                    // addOnSecondPass is true, we add up
+                    // term stats on the 2nd iteration:
                     boolean addOnSecondPass = random().nextBoolean();
 
                     //System.out.println("write isMerge=" + isMerge + " 2ndPass=" + addOnSecondPass);
@@ -1456,21 +1473,26 @@ public abstract class BasePostingsFormatTestCase extends LuceneTestCase {
                       String termString = term.utf8ToString();
 
                       // During merge we should only see terms
-                      // we had already seen during flush:
+                      // we had already seen during a
+                      // previous flush:
                       assertTrue(isMerge==false || termFreqs.containsKey(termString));
 
-                      if (isMerge == false && addOnSecondPass == false) {
-                        TermFreqs tf = termFreqs.get(termString);
-                        if (tf == null) {
-                          tf = new TermFreqs();
-                          termFreqs.put(termString, tf);
+                      if (isMerge == false) {
+                        if (addOnSecondPass == false) {
+                          TermFreqs tf = termFreqs.get(termString);
+                          if (tf == null) {
+                            tf = new TermFreqs();
+                            termFreqs.put(termString, tf);
+                          }
+                          tf.docFreq += docFreq;
+                          tf.totalTermFreq += totalTermFreq;
+                          sumDocFreq.addAndGet(docFreq);
+                          sumTotalTermFreq.addAndGet(totalTermFreq);
+                        } else if (termFreqs.containsKey(termString) == false) {
+                          // Add placeholder (2nd pass will
+                          // set its counts):
+                          termFreqs.put(termString, new TermFreqs());
                         }
-                        tf.docFreq += docFreq;
-                        tf.totalTermFreq += totalTermFreq;
-                        sumDocFreq.addAndGet(docFreq);
-                        sumTotalTermFreq.addAndGet(totalTermFreq);
-                      } else if (termFreqs.containsKey(termString) == false) {
-                        termFreqs.put(termString, new TermFreqs());
                       }
                     }
 
@@ -1503,10 +1525,7 @@ public abstract class BasePostingsFormatTestCase extends LuceneTestCase {
 
                         if (isMerge == false && addOnSecondPass) {
                           TermFreqs tf = termFreqs.get(term);
-                          if (tf == null) {
-                            tf = new TermFreqs();
-                            termFreqs.put(term, tf);
-                          }
+                          assert tf != null;
                           tf.docFreq += docFreq;
                           tf.totalTermFreq += totalTermFreq;
                           sumDocFreq.addAndGet(docFreq);