diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
index 26e7e3dbb6af..eaef6c68fd0c 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
@@ -475,7 +475,9 @@ public IndexWriterConfig setIndexSort(Sort sort) {
 
   @Override
   public String toString() {
-    StringBuilder sb = new StringBuilder(super.toString());
+    String superString = super.toString();
+    StringBuilder sb = new StringBuilder(superString.length() + 512);
+    sb.append(superString);
     sb.append("writer=").append(writer.get()).append("\n");
     return sb.toString();
   }