diff --git a/include/zeus/expected.hpp b/include/zeus/expected.hpp
index 556a561..0463be4 100644
--- a/include/zeus/expected.hpp
+++ b/include/zeus/expected.hpp
@@ -240,8 +240,15 @@ class unexpected
     friend constexpr void swap(unexpected &x, unexpected &y) noexcept(noexcept(x.swap(y))) { x.swap(y); }
 
     template<class E2>
-    friend constexpr bool operator==(const unexpected &lhs, const unexpected<E2> &rhs) noexcept(noexcept(lhs.error() == rhs.error()))
+    friend constexpr bool operator==(
+        const unexpected &lhs, const unexpected<E2> &rhs
+    ) noexcept(expected_detail::is_nothrow_equality_result_convertible_to_bool_v<E, E2>)
     {
+        // [expected.un.eq] Mandates
+        static_assert(
+            expected_detail::is_equality_result_convertible_to_bool_v<E, E2>,
+            "The expression lhs.error() == rhs.error() must be well-formed and its result must be convertible to bool"
+        );
         return lhs.error() == rhs.error();
     }
     template<class E2>