diff --git a/src/libstore/unix/build/derivation-builder.cc b/src/libstore/unix/build/derivation-builder.cc
index ce335ab19..932696029 100644
--- a/src/libstore/unix/build/derivation-builder.cc
+++ b/src/libstore/unix/build/derivation-builder.cc
@@ -107,17 +107,6 @@ protected:
      */
     const DerivationType derivationType;
 
-    /**
-     * Stuff we need to pass to initChild().
-     */
-    struct ChrootPath
-    {
-        Path source;
-        bool optional = false;
-    };
-
-    typedef std::map<Path, ChrootPath> PathsInChroot; // maps target path to source path
-
     typedef StringMap Environment;
     Environment env;
 
@@ -865,7 +854,7 @@ void DerivationBuilderImpl::startBuilder()
     processSandboxSetupMessages();
 }
 
-DerivationBuilderImpl::PathsInChroot DerivationBuilderImpl::getPathsInSandbox()
+PathsInChroot DerivationBuilderImpl::getPathsInSandbox()
 {
     PathsInChroot pathsInChroot;
 
diff --git a/src/libstore/unix/include/nix/store/build/derivation-builder.hh b/src/libstore/unix/include/nix/store/build/derivation-builder.hh
index a7ebb6c84..1d9b44163 100644
--- a/src/libstore/unix/include/nix/store/build/derivation-builder.hh
+++ b/src/libstore/unix/include/nix/store/build/derivation-builder.hh
@@ -11,6 +11,17 @@
 
 namespace nix {
 
+/**
+ * Stuff we need to pass to initChild().
+ */
+struct ChrootPath
+{
+    Path source;
+    bool optional = false;
+};
+
+typedef std::map<Path, ChrootPath> PathsInChroot; // maps target path to source path
+
 /**
  * Parameters by (mostly) `const` reference for `DerivationBuilder`.
  */