diff --git a/Sources/MessagePackExtension.swift b/Sources/MessagePackExtension.swift
index a5a6939..cd6af08 100644
--- a/Sources/MessagePackExtension.swift
+++ b/Sources/MessagePackExtension.swift
@@ -52,3 +52,8 @@ extension MessagePackExtension: MessagePackable {
         return try MessagePackType.ExtensionType.unpack(for: value)
     }
 }
+
+#if swift(>=5.7)
+extension MessagePackExtension: Sendable {
+}
+#endif
diff --git a/Sources/MessagePackTimestamp.swift b/Sources/MessagePackTimestamp.swift
index 94f2e7b..3d83f80 100644
--- a/Sources/MessagePackTimestamp.swift
+++ b/Sources/MessagePackTimestamp.swift
@@ -67,3 +67,8 @@ extension MessagePackTimestamp: MessagePackable {
         return try MessagePackTimestamp(extension: ext)
     }
 }
+
+#if swift(>=5.7)
+extension MessagePackTimestamp: Sendable {
+}
+#endif