diff --git a/lucene/src/test/org/apache/lucene/TestExternalCodecs.java b/lucene/src/test/org/apache/lucene/TestExternalCodecs.java
index e6390b154ed..298f7b7729f 100644
--- a/lucene/src/test/org/apache/lucene/TestExternalCodecs.java
+++ b/lucene/src/test/org/apache/lucene/TestExternalCodecs.java
@@ -484,7 +484,9 @@ public class TestExternalCodecs extends LuceneTestCase {
     public FieldsProducer fieldsProducer(SegmentReadState readState)
       throws IOException {
     
-      return state.get(readState.segmentInfo.name);
+      synchronized(state) {
+        return state.get(readState.segmentInfo.name);
+      }
     }
 
     @Override