diff --git a/src/libfetchers/include/nix/fetchers/input-cache.hh b/src/libfetchers/include/nix/fetchers/input-cache.hh
index 463927cea..4a6a1dff8 100644
--- a/src/libfetchers/include/nix/fetchers/input-cache.hh
+++ b/src/libfetchers/include/nix/fetchers/input-cache.hh
@@ -1,3 +1,5 @@
+#pragma once
+
 #include "nix/fetchers/fetchers.hh"
 
 namespace nix::fetchers {