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 88c668f4dd9..2ee380e3478 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
@@ -1475,7 +1475,9 @@ public abstract class BasePostingsFormatTestCase extends LuceneTestCase {
                     }
 
                     // Also test seeking the TermsEnum:
-                    for(String term : termFreqs.keySet()) {
+                    // must make copy because this very code sometimes makes puts while iterating?!
+                    Set<String> copy = new HashSet<String>(termFreqs.keySet());
+                    for(String term : copy) {
                       if (termsEnum.seekExact(new BytesRef(term))) {
                         if (random().nextBoolean()) {
                           docs = termsEnum.docs(null, docs, DocsEnum.FLAG_FREQS);