diff --git a/lucene/core/src/java/org/apache/lucene/index/FieldInfos.java b/lucene/core/src/java/org/apache/lucene/index/FieldInfos.java
index 5013ca755c3..ad1ae95a487 100644
--- a/lucene/core/src/java/org/apache/lucene/index/FieldInfos.java
+++ b/lucene/core/src/java/org/apache/lucene/index/FieldInfos.java
@@ -352,7 +352,7 @@ public class FieldInfos implements Iterable<FieldInfo> {
       this.softDeletesFieldName = softDeletesFieldName;
     }
 
-    void verifyFieldInfo(FieldInfo fi) {
+    synchronized void verifyFieldInfo(FieldInfo fi) {
       String fieldName = fi.getName();
       verifySoftDeletedFieldName(fieldName, fi.isSoftDeletesField());
       if (nameToNumber.containsKey(fieldName)) {