diff --git a/src/libstore/include/nix/store/build/derivation-builder.hh b/src/libstore/include/nix/store/build/derivation-builder.hh
index af84661e2..f28fd8e56 100644
--- a/src/libstore/include/nix/store/build/derivation-builder.hh
+++ b/src/libstore/include/nix/store/build/derivation-builder.hh
@@ -79,7 +79,7 @@ struct DerivationBuilderParams
      */
     const StorePathSet & inputPaths;
 
-    const std::map<std::string, InitialOutput> & initialOutputs;
+    const std::map<std::string, InitialOutput> initialOutputs;
 
     const BuildMode & buildMode;