diff --git a/src/libstore/builtins/buildenv.cc b/src/libstore/builtins/buildenv.cc
index 0ff0be3aa..22ed8d807 100644
--- a/src/libstore/builtins/buildenv.cc
+++ b/src/libstore/builtins/buildenv.cc
@@ -10,12 +10,16 @@
 
 namespace nix {
 
+namespace {
+
 struct State
 {
     std::map<Path, int> priorities;
     unsigned long symlinks = 0;
 };
 
+} // namespace
+
 /* For each activated package, create symlinks */
 static void createLinks(State & state, const Path & srcDir, const Path & dstDir, int priority)
 {