diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index a1cb162ee..0eb4bc79e 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -648,7 +648,8 @@ 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)