diff --git a/src/nix/env.cc b/src/nix/env.cc
index 0a211399a..a80bcda67 100644
--- a/src/nix/env.cc
+++ b/src/nix/env.cc
@@ -1,6 +1,7 @@
-#include <unordered_set>
 #include <queue>
 
+#include <boost/unordered/unordered_flat_set.hpp>
+
 #include "nix/cmd/command.hh"
 #include "nix/expr/eval.hh"
 #include "run.hh"