diff --git a/src/nix-find-roots/lib/find-roots.cc b/src/nix-find-roots/lib/find-roots.cc
index b2024439d..43465ed6b 100644
--- a/src/nix-find-roots/lib/find-roots.cc
+++ b/src/nix-find-roots/lib/find-roots.cc
@@ -12,6 +12,7 @@
 #include <vector>
 #include <algorithm>
 #include <fstream>
+#include <optional>
 
 #include "find-roots.hh"