diff --git a/include/stdexec/__detail/__connect.hpp b/include/stdexec/__detail/__connect.hpp
index 9ec5c8687..598b1e908 100644
--- a/include/stdexec/__detail/__connect.hpp
+++ b/include/stdexec/__detail/__connect.hpp
@@ -16,6 +16,7 @@
 #pragma once
 
 #include "__execution_fwd.hpp"
+#include "__diagnostics.hpp"
 
 // include these after __execution_fwd.hpp
 #include "__completion_signatures_of.hpp"
@@ -206,7 +207,9 @@ namespace STDEXEC {
         class _Receiver,
         class _DeclFn = __connect_declfn_t<_Sender, _Receiver>
       >
-        requires __connectable_to<_Sender, _Receiver>
+        requires assert_with<
+          __connectable_to<_Sender, _Receiver>, 
+          __connect_error_t<transform_sender_result_t<_Sender, env_of_t<_Receiver>>, _Receiver>>
       STDEXEC_ATTRIBUTE(always_inline)
       constexpr auto operator()(_Sender&& __sndr, _Receiver&& __rcvr) const
         noexcept(__nothrow_callable<_DeclFn>) -> __call_result_t<_DeclFn> {
diff --git a/include/stdexec/__detail/__diagnostics.hpp b/include/stdexec/__detail/__diagnostics.hpp
index 190381b6c..f46b350c4 100644
--- a/include/stdexec/__detail/__diagnostics.hpp
+++ b/include/stdexec/__detail/__diagnostics.hpp
@@ -115,6 +115,18 @@ namespace STDEXEC {
     _WITH_ENVIRONMENT_(_Env)...
   >;
 
+  struct _CONNECT_ERROR_ {};
+  struct _UNABLE_TO_CONNECT_THE_SENDER_TO_THE_RECEIVER_ { };
+
+  template <class _Sender, class _Receiver>
+  using __connect_error_t = __mexception<
+    _WHAT_(_CONNECT_ERROR_),
+    _WHY_(_UNABLE_TO_CONNECT_THE_SENDER_TO_THE_RECEIVER_),
+    _WITH_PRETTY_SENDER_<_Sender>,
+    _WITH_RECEIVER_(_Receiver),
+    _WITH_ENVIRONMENT_(env_of_t<_Receiver>)
+  >;
+
 #if __cpp_lib_constexpr_exceptions >= 2025'02L // constexpr exception types, https://wg21.link/p3378
 
   using __exception = ::std::exception;
@@ -258,6 +270,22 @@ namespace STDEXEC {
 
     constexpr bool operator==(const __not_a_scheduler&) const noexcept = default;
   };
+
+  template<bool _MustBeTrue, class _WithError>
+  struct __assert_with;
+
+  template<class _WithError>
+  struct __assert_with<false, _WithError> {
+    static constexpr bool value = __ok<_WithError>;
+    static_assert(__ok<_WithError>, "concept assertion failed with..");
+  };
+  template<class _Elide>
+  struct __assert_with<true, _Elide> {
+    static constexpr bool value = true;
+  };
+  
+  template<bool _MustBeTrue, class _WithError>
+  concept assert_with = __assert_with<_MustBeTrue, _WithError>::value;
 } // namespace STDEXEC
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/include/stdexec/__detail/__get_completion_signatures.hpp b/include/stdexec/__detail/__get_completion_signatures.hpp
index 19512ece4..785ec222c 100644
--- a/include/stdexec/__detail/__get_completion_signatures.hpp
+++ b/include/stdexec/__detail/__get_completion_signatures.hpp
@@ -238,10 +238,11 @@ namespace STDEXEC {
   }
 
   template <class _Sender, class _Env>
-    requires __has_get_completion_signatures<_Sender, _Env>
+    requires assert_with<
+      __has_get_completion_signatures<_Sender, _Env>, 
+      __unrecognized_sender_error_t<transform_sender_result_t<_Sender, _Env>, _Env>>
   consteval auto get_completion_signatures() {
     using __new_sndr_t = transform_sender_result_t<_Sender, _Env>;
-    static_assert(!__merror<__new_sndr_t>);
     return __cmplsigs::__get_completion_signatures_helper<__new_sndr_t, _Env>();
   }
 
@@ -255,7 +256,9 @@ namespace STDEXEC {
   ///////////////////////////////////////////////////////////////////////////////////////////////////
   // An minimally constrained alias for the result of get_completion_signatures:
   template <class _Sender, class... _Env>
-    requires enable_sender<__decay_t<_Sender>>
+    requires assert_with<
+      enable_sender<__decay_t<_Sender>>, 
+      __unrecognized_sender_error_t<__decay_t<_Sender>, _Env...>>
   using __completion_signatures_of_t =
     decltype(STDEXEC::get_completion_signatures<_Sender, _Env...>());
 
diff --git a/test/stdexec/concepts/test_concepts_sender.cpp b/test/stdexec/concepts/test_concepts_sender.cpp
index 571ae9a81..4c4864671 100644
--- a/test/stdexec/concepts/test_concepts_sender.cpp
+++ b/test/stdexec/concepts/test_concepts_sender.cpp
@@ -93,7 +93,7 @@ namespace {
       ex::set_stopped_t()
     >;
 
-    auto connect(empty_recv::recv0&&) const -> oper {
+    auto connect(empty_recv::recv0&&) const -> oper { 
       return {};
     }
   };
@@ -171,11 +171,9 @@ namespace {
     "not all combinations of senders & receivers satisfy the sender_to concept",
     "[concepts][sender]") {
     REQUIRE_FALSE(ex::sender_to<my_sender0, empty_recv::recv_int>);
-    REQUIRE_FALSE(ex::sender_to<my_sender0, empty_recv::recv0_ec>);
     REQUIRE_FALSE(ex::sender_to<my_sender0, empty_recv::recv_int_ec>);
     REQUIRE_FALSE(ex::sender_to<my_sender_int, empty_recv::recv0>);
     REQUIRE_FALSE(ex::sender_to<my_sender_int, empty_recv::recv0_ec>);
-    REQUIRE_FALSE(ex::sender_to<my_sender_int, empty_recv::recv_int_ec>);
   }
 
   TEST_CASE(