diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 830db4558..325ae792c 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -227,6 +227,7 @@ public:
     template<class T, typename... Args>
     [[nodiscard, gnu::noinline]]
     EvalErrorBuilder<T> & error(const Args & ... args) {
+        // `EvalErrorBuilder::debugThrow` performs the corresponding `delete`.
         return *new EvalErrorBuilder<T>(*this, args...);
     }