diff --git a/src/libexpr/flake/lockfile.cc b/src/libexpr/flake/lockfile.cc
index 6a64f113e..3d796eadc 100644
--- a/src/libexpr/flake/lockfile.cc
+++ b/src/libexpr/flake/lockfile.cc
@@ -111,8 +111,7 @@ nlohmann::json LockedInput::toJson() const
 
 StorePath LockedInput::computeStorePath(Store & store) const
 {
-    assert(info.narHash);
-    return store.makeFixedOutputPath(true, info.narHash, "source");
+    return info.computeStorePath(store);
 }
 
 LockedInputs::LockedInputs(const nlohmann::json & json)
diff --git a/src/libexpr/flake/lockfile.hh b/src/libexpr/flake/lockfile.hh
index 6333a6875..6203c40cb 100644
--- a/src/libexpr/flake/lockfile.hh
+++ b/src/libexpr/flake/lockfile.hh
@@ -11,6 +11,8 @@ struct StorePath;
 
 namespace nix::flake {
 
+using namespace fetchers;
+
 typedef std::vector<FlakeId> InputPath;
 
 struct LockedInput;
diff --git a/src/libstore/fetchers/fetchers.cc b/src/libstore/fetchers/fetchers.cc
index 89cc9e774..373786f38 100644
--- a/src/libstore/fetchers/fetchers.cc
+++ b/src/libstore/fetchers/fetchers.cc
@@ -107,4 +107,10 @@ std::shared_ptr<const Input> Input::applyOverrides(
     return shared_from_this();
 }
 
+StorePath TreeInfo::computeStorePath(Store & store) const
+{
+    assert(narHash);
+    return store.makeFixedOutputPath(true, narHash, "source");
+}
+
 }
diff --git a/src/libstore/fetchers/tree-info.hh b/src/libstore/fetchers/tree-info.hh
index 28ead8588..02e92759b 100644
--- a/src/libstore/fetchers/tree-info.hh
+++ b/src/libstore/fetchers/tree-info.hh
@@ -1,6 +1,10 @@
 #pragma once
 
-namespace nix {
+#include "path.hh"
+
+namespace nix { class Store; }
+
+namespace nix::fetchers {
 
 struct TreeInfo
 {
@@ -15,6 +19,8 @@ struct TreeInfo
             && revCount == other.revCount
             && lastModified == other.lastModified;
     }
+
+    StorePath computeStorePath(Store & store) const;
 };
 
 }