diff --git a/java/com/google/gerrit/server/restapi/change/RelatedChangesSorter.java b/java/com/google/gerrit/server/restapi/change/RelatedChangesSorter.java
index 8040847e72..af65483f0b 100644
--- a/java/com/google/gerrit/server/restapi/change/RelatedChangesSorter.java
+++ b/java/com/google/gerrit/server/restapi/change/RelatedChangesSorter.java
@@ -16,6 +16,7 @@ package com.google.gerrit.server.restapi.change;
 
 import static com.google.common.base.Preconditions.checkArgument;
 import static java.util.Objects.requireNonNull;
+import static java.util.stream.Collectors.toMap;
 
 import com.google.auto.value.AutoValue;
 import com.google.common.annotations.VisibleForTesting;
@@ -76,7 +77,15 @@ class RelatedChangesSorter {
     // Map of all patch sets, keyed by commit SHA-1.
     Map<ObjectId, PatchSetData> byId = collectById(in);
     PatchSetData start = byId.get(startPs.commitId());
-    checkArgument(start != null, "%s not found in %s", startPs, in);
+    requireNonNull(
+        start,
+        () ->
+            String.format(
+                "commit %s of patch set %s not found in %s",
+                startPs.commitId().name(),
+                startPs.id(),
+                byId.entrySet().stream()
+                    .collect(toMap(e -> e.getKey().name(), e -> e.getValue().patchSet().id()))));
 
     // Map of patch set -> immediate parent.
     ListMultimap<PatchSetData, PatchSetData> parents =