diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index ed9af78b3..e9d9d02a4 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -673,7 +673,8 @@ std::vector<std::string> AttrCursor::getListOfStrings()
     for (auto & elem : v.listItems())
         res.push_back(std::string(root->state.forceStringNoCtx(*elem)));
 
-    cachedValue = {root->db->setListOfStrings(getKey(), res), res};
+    if (root->db)
+        cachedValue = {root->db->setListOfStrings(getKey(), res), res};
 
     return res;
 }
diff --git a/tests/build.sh b/tests/build.sh
index ff16d1603..fc6825e25 100644
--- a/tests/build.sh
+++ b/tests/build.sh
@@ -56,6 +56,13 @@ nix build -f multiple-outputs.nix --json 'e^*' --no-link | jq --exit-status '
     (.outputs | keys == ["a", "b", "c"]))
 '
 
+# Make sure that `--impure` works (regression test for https://github.com/NixOS/nix/issues/6488)
+nix build --impure -f multiple-outputs.nix --json e --no-link | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-e.drv")) and
+    (.outputs | keys == ["a", "b"]))
+'
+
 testNormalization () {
     clearStore
     outPath=$(nix-build ./simple.nix --no-out-link)
diff --git a/tests/lang/eval-okay-fromjson.nix b/tests/lang/eval-okay-fromjson.nix
index 102ee82b5..e1c0f86cc 100644
--- a/tests/lang/eval-okay-fromjson.nix
+++ b/tests/lang/eval-okay-fromjson.nix
@@ -1,36 +1,35 @@
-# RFC 7159, section 13.
 builtins.fromJSON
   ''
     {
-      "Image": {
-          "Width":  800,
-          "Height": 600,
-          "Title":  "View from 15th Floor",
-          "Thumbnail": {
-              "Url":    "http://www.example.com/image/481989943",
-              "Height": 125,
-              "Width":  100
+      "Video": {
+          "Title":  "The Penguin Chronicles",
+          "Width":  1920,
+          "Height": 1080,
+          "EmbeddedData": [3.14159, 23493,null, true  ,false, -10],
+          "Thumb": {
+              "Url":    "http://www.example.com/video/5678931",
+              "Width":  200,
+              "Height": 250
           },
-          "Animated" : false,
-          "IDs": [116, 943, 234, 38793, true  ,false,null, -100],
-          "Latitude":  37.7668,
-          "Longitude": -122.3959
+          "Subtitle" : false,
+          "Latitude":  46.2051,
+          "Longitude": 6.0723
         }
     }
   ''
 ==
-  { Image =
-    { Width = 800;
-      Height = 600;
-      Title = "View from 15th Floor";
-      Thumbnail =
-        { Url = http://www.example.com/image/481989943;
-          Height = 125;
-          Width = 100;
+  { Video =
+    { Title = "The Penguin Chronicles";
+      Width = 1920;
+      Height = 1080;
+      EmbeddedData = [ 3.14159 23493 null true false (0-10) ];
+      Thumb =
+        { Url = "http://www.example.com/video/5678931";
+          Width = 200;
+          Height = 250;
         };
-      Animated = false;
-      IDs = [ 116 943 234 38793 true false null (0-100) ];
-      Latitude = 37.7668;
-      Longitude = -122.3959;
+      Subtitle = false;
+      Latitude = 46.2051;
+      Longitude = 6.0723;
     };
   }