diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 1473a7660..049f2e5a2 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -240,7 +240,7 @@ EvalState::EvalState(
 
         auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
         if (settings.pureEval || store->storeDir != realStoreDir) {
-            accessor = settings.pureEval ? storeFS : makeUnionSourceAccessor({accessor, storeFS});
+            accessor = settings.pureEval ? storeFS.cast<SourceAccessor>() : makeUnionSourceAccessor({accessor, storeFS});
         }
 
         /* Apply access control if needed. */
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index b841f0bc6..66ff7d6ea 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/src/libexpr/include/nix/expr/eval.hh
@@ -48,6 +48,7 @@ class StorePath;
 struct SingleDerivedPath;
 enum RepairFlag : bool;
 struct MemorySourceAccessor;
+struct MountedSourceAccessor;
 
 namespace eval_cache {
 class EvalCache;
@@ -319,7 +320,7 @@ public:
     /**
      * The accessor corresponding to `store`.
      */
-    const ref<SourceAccessor> storeFS;
+    const ref<MountedSourceAccessor> storeFS;
 
     /**
      * The accessor for the root filesystem.