diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 173058d1b..5e86b5269 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -544,7 +544,8 @@ void DerivationGoal::inputsRealised()
                    However, the impure derivations feature still relies on this
                    fragile way of doing things, because its builds do not have
                    a representation in the store, which is a usability problem
-                   in itself */
+                   in itself. When implementing this logic entirely with lookups
+                   make sure that they're cached. */
                 if (auto outPath = get(inputDrvOutputs, { depDrvPath, j })) {
                     worker.store.computeFSClosure(*outPath, inputPaths);
                 }