diff --git a/include/rvarago/optional_extras/algorithm.hpp b/include/rvarago/optional_extras/algorithm.hpp
index 8098abd..71cc8a8 100644
--- a/include/rvarago/optional_extras/algorithm.hpp
+++ b/include/rvarago/optional_extras/algorithm.hpp
@@ -105,15 +105,14 @@ append(std::optional<T> opt_lhs, std::optional<T> opt_rhs,
 
 template <typename T, std::regular_invocable<> NullaryFunction,
           std::regular_invocable<T> UnaryFunction>
-  requires std::same_as<std::invoke_result_t<NullaryFunction>,
-                        std::invoke_result_t<UnaryFunction, T &&>>
 constexpr auto fold(
     std::optional<T> opt, NullaryFunction &&when_empty,
     UnaryFunction &&
         when_engaged) noexcept(std::is_nothrow_move_constructible_v<T> &&
                                std::is_nothrow_invocable_v<NullaryFunction> &&
                                std::is_nothrow_invocable_v<UnaryFunction, T &&>)
-    -> std::invoke_result_t<NullaryFunction> {
+    -> std::common_type_t<std::invoke_result_t<NullaryFunction>,
+                          std::invoke_result_t<UnaryFunction, T &&>> {
   if (opt) {
     return std::invoke(std::forward<UnaryFunction>(when_engaged),
                        std::move(*opt));
diff --git a/test/algorithm_test.cpp b/test/algorithm_test.cpp
index 7ab2dbc..5f1d3d7 100644
--- a/test/algorithm_test.cpp
+++ b/test/algorithm_test.cpp
@@ -92,4 +92,8 @@ TEST_CASE("When engaged transform it otherwise invoke default", "[fold]") {
 
   STATIC_REQUIRE(optx::fold(none<int>, zero, bail<int, int>) == 0);
   STATIC_REQUIRE(optx::fold(optional{1}, bail<int>, inc) == 2);
+
+  // implicit return-type conversion.
+  STATIC_REQUIRE(
+      optx::fold(none<int>, [] { return true; }, bail<int, int>) == 1);
 }