diff --git a/kernel/hashlib.h b/kernel/hashlib.h
index 6e3eb32a4..4144d701d 100644
--- a/kernel/hashlib.h
+++ b/kernel/hashlib.h
@@ -14,6 +14,7 @@
 
 #include <stdexcept>
 #include <algorithm>
+#include <optional>
 #include <string>
 #include <variant>
 #include <vector>