diff --git a/doc/src/sgml/ref/repack.sgml b/doc/src/sgml/ref/repack.sgml
index 3ba037848f6..346cba89c90 100644
--- a/doc/src/sgml/ref/repack.sgml
+++ b/doc/src/sgml/ref/repack.sgml
@@ -384,6 +384,12 @@ REPACK [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] USING
     on the table.
    </para>
 
+   <para>
+    <command>REPACK</command> refuses to process a table on which an invalid
+    index exists.  Such indexes must be dropped or reindexed by the user ahead
+    of time.
+   </para>
+
    <para>
     While <command>REPACK</command> is running, the <xref
     linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 609e2c1d77e..33d243424a3 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -886,13 +886,15 @@ mark_index_clustered(Relation rel, Oid indexOid, bool is_internal)
 /*
  * check_index_requirements: verify index state on relation being processed
  *
- * Throw an error if any !indisready indexes are found.
+ * Throw an error if any incompletely-built indexes are found.
  *
  * Indexes that are not ready for inserts, such as ones left behind by failed
- * CREATE INDEX CONCURRENTLY, are not maintained by DML.  In some cases they
- * may fail to build altogether.  Throwing an error here forces the user to
- * take action on these indexes separately from the table reconstruction,
- * which prevents perpetuating them for no reason.
+ * CREATE INDEX CONCURRENTLY, are not maintained by DML.  Indexes that aren't
+ * marked valid could have been in the middle of validation when their build
+ * failed, and thus it's not certain that they could be built.  In both cases,
+ * attempting to rebuild may fail altogether.  Throwing an error here forces
+ * the user to take action on these indexes separately from the table
+ * reconstruction, which prevents perpetuating them for no reason.
  */
 static void
 check_index_requirements(Relation rel, RepackCommand cmd)
@@ -920,9 +922,8 @@ check_index_requirements(Relation rel, RepackCommand cmd)
 	{
 		Form_pg_index index = (Form_pg_index) GETSTRUCT(htup);
 
-		if (!index->indisready)
+		if (!index->indisvalid)
 		{
-			Assert(!index->indisvalid);
 			if (num_invalid_idxs == 0)
 				appendStringInfo(&dest, _("\"%s\""), get_rel_name(index->indexrelid));
 			else