diff --git a/src/libstore/build/derivation-building-goal.cc b/src/libstore/build/derivation-building-goal.cc
index 77ab23b4c..fd85a066d 100644
--- a/src/libstore/build/derivation-building-goal.cc
+++ b/src/libstore/build/derivation-building-goal.cc
@@ -470,6 +470,11 @@ void DerivationBuildingGoal::started()
 
 Goal::Co DerivationBuildingGoal::tryToBuild()
 {
+    /**
+     * Activity that denotes waiting for a lock.
+     */
+    std::unique_ptr<Activity> actLock;
+
     bool useHook;
 
     while (true) {
diff --git a/src/libstore/include/nix/store/build/derivation-building-goal.hh b/src/libstore/include/nix/store/build/derivation-building-goal.hh
index 162cf14ad..dd8b27dc2 100644
--- a/src/libstore/include/nix/store/build/derivation-building-goal.hh
+++ b/src/libstore/include/nix/store/build/derivation-building-goal.hh
@@ -92,11 +92,6 @@ struct DerivationBuildingGoal : public Goal
 
     std::unique_ptr<Activity> act;
 
-    /**
-     * Activity that denotes waiting for a lock.
-     */
-    std::unique_ptr<Activity> actLock;
-
     std::map<ActivityId, Activity> builderActivities;
 
     /**