diff --git a/jasmin/core/BUILD b/jasmin/core/BUILD
index 3f5ab2d..3f9f655 100644
--- a/jasmin/core/BUILD
+++ b/jasmin/core/BUILD
@@ -50,6 +50,7 @@ cc_library(
     visibility = ["//visibility:public"],
     deps = [
         "//jasmin/core/internal:function_forward",
+        "@nth_cc//nth/debug",
         "@nth_cc//nth/io/deserialize",
         "@nth_cc//nth/io/reader",
         "@nth_cc//nth/io/serialize",
diff --git a/jasmin/core/function.h b/jasmin/core/function.h
index de632bd..10fe10b 100644
--- a/jasmin/core/function.h
+++ b/jasmin/core/function.h
@@ -36,6 +36,7 @@ struct Function<void> : internal::FunctionBase {
   template <nth::io::serializer_with_context<FunctionRegistry> S>
   friend nth::io::serializer_result_type<S> NthSerialize(S &s,
                                                          Function const *f) {
+    if (not f) { NTH_UNIMPLEMENTED(); }
     return nth::io::serialize(s, s.context(nth::type<FunctionRegistry>).get(f));
   }
 
@@ -54,7 +55,8 @@ struct Function<void> : internal::FunctionBase {
     if (not result) { return result; }
     auto &registry = d.context(nth::type<FunctionRegistry>);
     fn             = registry[id];
-    return result_type(true);
+    if (fn == nullptr) { NTH_UNIMPLEMENTED(); }
+    return result_type(fn != nullptr);
   }
 
  protected:
diff --git a/jasmin/core/function_registry.cc b/jasmin/core/function_registry.cc
index e303555..703b446 100644
--- a/jasmin/core/function_registry.cc
+++ b/jasmin/core/function_registry.cc
@@ -20,7 +20,6 @@ FunctionIdentifier FunctionRegistry::get(Function<> const *f) {
 Function<> const *FunctionRegistry::operator[](FunctionIdentifier id) const {
   if (id.index_ >= registered_functions_.size()) { return nullptr; }
   auto [f, pf] = registered_functions_.from_index(id.index_);
-  if (pf != id.fragment_) { return nullptr; }
   return f;
 }
 
diff --git a/jasmin/core/program_fragment.h b/jasmin/core/program_fragment.h
index a7edb39..716bc6e 100644
--- a/jasmin/core/program_fragment.h
+++ b/jasmin/core/program_fragment.h
@@ -117,8 +117,7 @@ struct ProgramFragment : internal::ProgramFragmentBase {
       }
 
       auto [iter, inserted] = p.functions_.try_emplace(name);
-      if (not inserted) { return result_type(false); }
-      registry.register_function(p, iter->second);
+      if (inserted) { registry.register_function(p, iter->second); }
       fns.push_back(&iter->second);
     }