diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index 422f1c658e4..cead9af537e 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -245,6 +245,9 @@ Other Changes
 
 * SOLR-9801: Upgrade jetty to 9.3.14.v20161028 (shalin)
 
+* SOLR-9783: (Search|Top)Group[s]ShardResponseProcessor.process: turned sortWithinGroup null check into assert.
+  (Christine Poerschke)
+
 ==================  6.3.0 ==================
 
 Consult the LUCENE_CHANGES.txt file for additional, low level, changes in this release.
diff --git a/solr/core/src/java/org/apache/solr/search/grouping/distributed/command/QueryCommand.java b/solr/core/src/java/org/apache/solr/search/grouping/distributed/command/QueryCommand.java
index 86fe729447e..afb8ba78a9c 100644
--- a/solr/core/src/java/org/apache/solr/search/grouping/distributed/command/QueryCommand.java
+++ b/solr/core/src/java/org/apache/solr/search/grouping/distributed/command/QueryCommand.java
@@ -124,7 +124,7 @@ public class QueryCommand implements Command<QueryCommandResult> {
 
   @Override
   public List<Collector> create() throws IOException {
-    if (sort == null || sort == Sort.RELEVANCE) {
+    if (sort == null || sort.equals(Sort.RELEVANCE)) {
       collector = TopScoreDocCollector.create(docsToCollect);
     } else {
       collector = TopFieldCollector.create(sort, docsToCollect, true, needScores, needScores);
diff --git a/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/SearchGroupShardResponseProcessor.java b/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/SearchGroupShardResponseProcessor.java
index 18b0de54a44..0acd6f90e27 100644
--- a/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/SearchGroupShardResponseProcessor.java
+++ b/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/SearchGroupShardResponseProcessor.java
@@ -52,9 +52,7 @@ public class SearchGroupShardResponseProcessor implements ShardResponseProcessor
     Sort groupSort = rb.getGroupingSpec().getGroupSort();
     final String[] fields = rb.getGroupingSpec().getFields();
     Sort sortWithinGroup = rb.getGroupingSpec().getSortWithinGroup();
-    if (sortWithinGroup == null) { // TODO prevent it from being null in the first place
-      sortWithinGroup = Sort.RELEVANCE;
-    }
+    assert sortWithinGroup != null;
 
     final Map<String, List<Collection<SearchGroup<BytesRef>>>> commandSearchGroups = new HashMap<>(fields.length, 1.0f);
     final Map<String, Map<SearchGroup<BytesRef>, Set<String>>> tempSearchGroupToShards = new HashMap<>(fields.length, 1.0f);
diff --git a/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/TopGroupsShardResponseProcessor.java b/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/TopGroupsShardResponseProcessor.java
index 688a6c37011..3610a383ccb 100644
--- a/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/TopGroupsShardResponseProcessor.java
+++ b/solr/core/src/java/org/apache/solr/search/grouping/distributed/responseprocessor/TopGroupsShardResponseProcessor.java
@@ -61,9 +61,7 @@ public class TopGroupsShardResponseProcessor implements ShardResponseProcessor {
     String[] fields = rb.getGroupingSpec().getFields();
     String[] queries = rb.getGroupingSpec().getQueries();
     Sort sortWithinGroup = rb.getGroupingSpec().getSortWithinGroup();
-    if (sortWithinGroup == null) { // TODO prevent it from being null in the first place
-      sortWithinGroup = Sort.RELEVANCE;
-    }
+    assert sortWithinGroup != null;
 
     // If group.format=simple group.offset doesn't make sense
     int groupOffsetDefault;