diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 9b6bea99..d906a69 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -266,6 +266,8 @@
 
 * LUCENE-9773: Upgrade icu to 68.2 (Robert Muir)
 
+* LUCENE-9822: Add assertion to PFOR exception encoding, documenting the BLOCK_SIZE assumption. (Greg Miller)
+
 ======================= Lucene 8.9.0 =======================
 
 API Changes
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene90/PForUtil.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene90/PForUtil.java
index 17af2e5..4376036 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene90/PForUtil.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene90/PForUtil.java
@@ -37,6 +37,7 @@
   private final ForUtil forUtil;
 
   PForUtil(ForUtil forUtil) {
+    assert ForUtil.BLOCK_SIZE <= 256 : "blocksize must fit in one byte. got " + ForUtil.BLOCK_SIZE;
     this.forUtil = forUtil;
   }