diff --git a/Projects/Server.Tests/Tests/Serialization/SerializationChunkSourceTests.cs b/Projects/Server.Tests/Tests/Serialization/SerializationChunkSourceTests.cs
index 3f315e8d9..1cdb08188 100644
--- a/Projects/Server.Tests/Tests/Serialization/SerializationChunkSourceTests.cs
+++ b/Projects/Server.Tests/Tests/Serialization/SerializationChunkSourceTests.cs
@@ -4,6 +4,10 @@ using Xunit;
 
 namespace Server.Tests;
 
+// Constructing a persistence mutates the static Persistence registry (an unsynchronized
+// SortedSet); every test that does so must share the sequential collection or parallel
+// collections corrupt the tree.
+[Collection("Sequential Server Tests")]
 public class SerializationChunkSourceTests
 {
     private class TestEntity : IGenericSerializable
diff --git a/Projects/Server.Tests/Tests/Serialization/ShadowDictionaryEntriesTests.cs b/Projects/Server.Tests/Tests/Serialization/ShadowDictionaryEntriesTests.cs
index 6f5661f5a..315a58900 100644
--- a/Projects/Server.Tests/Tests/Serialization/ShadowDictionaryEntriesTests.cs
+++ b/Projects/Server.Tests/Tests/Serialization/ShadowDictionaryEntriesTests.cs
@@ -4,6 +4,10 @@ using Xunit;
 
 namespace Server.Tests;
 
+// Constructing a persistence mutates the static Persistence registry (an unsynchronized
+// SortedSet); every test that does so must share the sequential collection or parallel
+// collections corrupt the tree.
+[Collection("Sequential Server Tests")]
 public class ShadowDictionaryEntriesTests
 {
     private class TestEntity : ISerializable