diff --git a/runtime/vm/code_descriptors_test.cc b/runtime/vm/code_descriptors_test.cc
index c5faf2ac17c..8c192b5705c 100644
--- a/runtime/vm/code_descriptors_test.cc
+++ b/runtime/vm/code_descriptors_test.cc
@@ -36,7 +36,7 @@ static Dart_NativeFunction native_resolver(Dart_Handle name,
                                            bool* auto_setup_scope) {
   ASSERT(auto_setup_scope);
   *auto_setup_scope = false;
-  return reinterpret_cast<Dart_NativeFunction>(&NativeFunc);
+  return NativeFunc;
 }
 
 TEST_CASE(StackMapGC) {
diff --git a/runtime/vm/compiler/backend/redundancy_elimination_test.cc b/runtime/vm/compiler/backend/redundancy_elimination_test.cc
index 74cf38ec22a..01c3fbd3c62 100644
--- a/runtime/vm/compiler/backend/redundancy_elimination_test.cc
+++ b/runtime/vm/compiler/backend/redundancy_elimination_test.cc
@@ -33,7 +33,7 @@ static Dart_NativeFunction NoopNativeLookup(Dart_Handle name,
                                             bool* auto_setup_scope) {
   ASSERT(auto_setup_scope != nullptr);
   *auto_setup_scope = false;
-  return reinterpret_cast<Dart_NativeFunction>(&NoopNative);
+  return NoopNative;
 }
 
 // Flatten all non-captured LocalVariables from the given scope and its children
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index e867ce4f3aa..e2840babd9a 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -653,23 +653,29 @@ bool Api::GetNativeFieldsOfArgument(NativeArguments* arguments,
                                     intptr_t* field_values) {
   NoSafepointScope no_safepoint_scope;
   ObjectPtr raw_obj = arguments->NativeArgAt(arg_index);
-  if (raw_obj->IsHeapObject()) {
-    intptr_t cid = raw_obj->GetClassId();
-    if (cid >= kNumPredefinedCids) {
-      TypedDataPtr native_fields = *reinterpret_cast<TypedDataPtr*>(
-          ObjectLayout::ToAddr(raw_obj) + sizeof(ObjectLayout));
-      if (native_fields == TypedData::null()) {
-        memset(field_values, 0, (num_fields * sizeof(field_values[0])));
-      } else if (num_fields == Smi::Value(native_fields->ptr()->length_)) {
-        intptr_t* native_values =
-            bit_cast<intptr_t*, uint8_t*>(native_fields->ptr()->data());
-        memmove(field_values, native_values,
-                (num_fields * sizeof(field_values[0])));
-      }
-      return true;
-    }
+  intptr_t cid = raw_obj->GetClassIdMayBeSmi();
+  int class_num_fields = arguments->thread()
+                             ->isolate()
+                             ->class_table()
+                             ->At(cid)
+                             ->ptr()
+                             ->num_native_fields_;
+  if (num_fields != class_num_fields) {
+    // No native fields or mismatched native field count.
+    return false;
   }
-  return false;
+  TypedDataPtr native_fields = *reinterpret_cast<TypedDataPtr*>(
+      ObjectLayout::ToAddr(raw_obj) + sizeof(ObjectLayout));
+  if (native_fields == TypedData::null()) {
+    // Native fields not initialized.
+    memset(field_values, 0, (num_fields * sizeof(field_values[0])));
+    return true;
+  }
+  ASSERT(class_num_fields == Smi::Value(native_fields->ptr()->length_));
+  intptr_t* native_values =
+      reinterpret_cast<intptr_t*>(native_fields->ptr()->data());
+  memmove(field_values, native_values, (num_fields * sizeof(field_values[0])));
+  return true;
 }
 
 void Api::SetWeakHandleReturnValue(NativeArguments* args,
diff --git a/runtime/vm/dart_api_impl_test.cc b/runtime/vm/dart_api_impl_test.cc
index 2c2a4eb4336..08b0c7943b0 100644
--- a/runtime/vm/dart_api_impl_test.cc
+++ b/runtime/vm/dart_api_impl_test.cc
@@ -538,7 +538,7 @@ static Dart_NativeFunction CurrentStackTraceNativeLookup(
     bool* auto_setup_scope) {
   ASSERT(auto_setup_scope != NULL);
   *auto_setup_scope = true;
-  return reinterpret_cast<Dart_NativeFunction>(&CurrentStackTraceNative);
+  return CurrentStackTraceNative;
 }
 
 TEST_CASE(DartAPI_CurrentStackTraceInfo) {
@@ -687,7 +687,7 @@ static Dart_NativeFunction PropagateError_native_lookup(
     bool* auto_setup_scope) {
   ASSERT(auto_setup_scope != NULL);
   *auto_setup_scope = true;
-  return reinterpret_cast<Dart_NativeFunction>(&PropagateErrorNative);
+  return PropagateErrorNative;
 }
 
 TEST_CASE(DartAPI_PropagateCompileTimeError) {
@@ -4995,7 +4995,7 @@ static Dart_NativeFunction native_field_lookup(Dart_Handle name,
                                                bool* auto_setup_scope) {
   ASSERT(auto_setup_scope != NULL);
   *auto_setup_scope = false;
-  return reinterpret_cast<Dart_NativeFunction>(&NativeFieldLookup);
+  return NativeFieldLookup;
 }
 
 TEST_CASE(DartAPI_InjectNativeFields2) {
@@ -5139,6 +5139,15 @@ void TestNativeFieldsAccess_access(Dart_NativeArguments args) {
   EXPECT_EQ(0, field_values[1]);
 }
 
+void TestNativeFieldsAccess_invalidAccess(Dart_NativeArguments args) {
+  intptr_t field_values[kTestNumNativeFields];
+  Dart_Handle result = Dart_GetNativeFieldsOfArgument(
+      args, 0, kTestNumNativeFields, field_values);
+  EXPECT_ERROR(result,
+               "Dart_GetNativeFieldsOfArgument: "
+               "expected 0 'num_fields' but was passed in 2");
+}
+
 static Dart_NativeFunction TestNativeFieldsAccess_lookup(Dart_Handle name,
                                                          int argument_count,
                                                          bool* auto_scope) {
@@ -5152,10 +5161,12 @@ static Dart_NativeFunction TestNativeFieldsAccess_lookup(Dart_Handle name,
   const char* function_name = obj.ToCString();
   ASSERT(function_name != NULL);
   if (strcmp(function_name, "TestNativeFieldsAccess_init") == 0) {
-    return reinterpret_cast<Dart_NativeFunction>(&TestNativeFieldsAccess_init);
+    return TestNativeFieldsAccess_init;
   } else if (strcmp(function_name, "TestNativeFieldsAccess_access") == 0) {
-    return reinterpret_cast<Dart_NativeFunction>(
-        &TestNativeFieldsAccess_access);
+    return TestNativeFieldsAccess_access;
+  } else if (strcmp(function_name, "TestNativeFieldsAccess_invalidAccess") ==
+             0) {
+    return TestNativeFieldsAccess_invalidAccess;
   } else {
     return NULL;
   }
@@ -5178,10 +5189,15 @@ TEST_CASE(DartAPI_TestNativeFieldsAccess) {
           "  int%s accessNativeFlds(int%s i) native "
           "'TestNativeFieldsAccess_access';\n"
           "}\n"
+          "class NoNativeFields {\n"
+          "  int neitherATypedDataNorNull = 0;\n"
+          "  invalidAccess() native 'TestNativeFieldsAccess_invalidAccess';\n"
+          "}\n"
           "NativeFields testMain() {\n"
           "  NativeFields obj = new NativeFields(10, 20);\n"
           "  obj.initNativeFlds();\n"
           "  obj.accessNativeFlds(null);\n"
+          "  new NoNativeFields().invalidAccess();\n"
           "  return obj;\n"
           "}\n",
           nullable_tag, nullable_tag, nullable_tag, nullable_tag),
@@ -6394,7 +6410,7 @@ static Dart_NativeFunction native_lookup(Dart_Handle name,
                                          bool* auto_setup_scope) {
   ASSERT(auto_setup_scope != NULL);
   *auto_setup_scope = true;
-  return reinterpret_cast<Dart_NativeFunction>(&ExceptionNative);
+  return ExceptionNative;
 }
 
 TEST_CASE(DartAPI_ThrowException) {
@@ -6557,9 +6573,9 @@ static Dart_NativeFunction native_args_lookup(Dart_Handle name,
   const char* function_name = obj.ToCString();
   ASSERT(function_name != NULL);
   if (strcmp(function_name, "NativeArgument_Create") == 0) {
-    return reinterpret_cast<Dart_NativeFunction>(&NativeArgumentCreate);
+    return NativeArgumentCreate;
   } else if (strcmp(function_name, "NativeArgument_Access") == 0) {
-    return reinterpret_cast<Dart_NativeFunction>(&NativeArgumentAccess);
+    return NativeArgumentAccess;
   }
   return NULL;
 }
@@ -6618,7 +6634,7 @@ static Dart_NativeFunction gnac_lookup(Dart_Handle name,
                                        bool* auto_setup_scope) {
   ASSERT(auto_setup_scope != NULL);
   *auto_setup_scope = true;
-  return reinterpret_cast<Dart_NativeFunction>(&NativeArgumentCounter);
+  return NativeArgumentCounter;
 }
 
 TEST_CASE(DartAPI_GetNativeArgumentCount) {
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 499dba8ac12..ccf4d45393e 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -806,6 +806,7 @@ class ClassLayout : public ObjectLayout {
   friend class SnapshotReader;
   friend class InstanceSerializationCluster;
   friend class CidRewriteVisitor;
+  friend class Api;
 };
 
 class PatchClassLayout : public ObjectLayout {