diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index 1fa9aee4e5d..225113af4e4 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -319,6 +319,7 @@ void InstanceMorpher::CreateMorphedCopies(Become* become) {
 
     // We also forward Enum.values. No filler is needed because arrays never
     // change shape.
+    ASSERT(old_values.ptr() != new_values.ptr());
     become->Add(old_values, new_values);
 
 #if defined(DEBUG)
diff --git a/runtime/vm/isolate_reload_test.cc b/runtime/vm/isolate_reload_test.cc
index 829d4d6d148..0fd416dcb23 100644
--- a/runtime/vm/isolate_reload_test.cc
+++ b/runtime/vm/isolate_reload_test.cc
@@ -5196,6 +5196,35 @@ TEST_CASE(IsolateReload_ImplicitGetterWithLoadGuard) {
   EXPECT_STREQ("y: 3, z: 8208", SimpleInvokeStr(lib1, "main"));
 }
 
+// Regression test for https://github.com/dart-lang/sdk/issues/51835
+TEST_CASE(IsolateReload_EnumInMainLibraryModified) {
+  const char* kScript =
+      "enum Bar { bar }\n"
+      "class Foo { int? a; toString() => 'foo'; }"
+      "main() {\n"
+      "  return Foo().toString();\n"
+      "}\n";
+
+  Dart_Handle lib = TestCase::LoadTestScript(kScript, NULL);
+  EXPECT_VALID(lib);
+  EXPECT_VALID(Dart_FinalizeAllClasses());
+  EXPECT_STREQ("foo", SimpleInvokeStr(lib, "main"));
+
+  const char* kReloadScript =
+      "enum Bar { bar }\n"
+      "class Foo { int? a; String? b; toString() => 'foo'; }"
+      "main() {\n"
+      "  return Foo().toString();\n"
+      "}\n";
+
+  lib = TestCase::ReloadTestScript(kReloadScript);
+  EXPECT_VALID(lib);
+  Dart_SetFileModifiedCallback(NULL);
+
+  // Modification of an imported library propagates to the importing library.
+  EXPECT_STREQ("foo", SimpleInvokeStr(lib, "main"));
+}
+
 #endif  // !defined(PRODUCT) && !defined(DART_PRECOMPILED_RUNTIME)
 
 }  // namespace dart
diff --git a/runtime/vm/object_reload.cc b/runtime/vm/object_reload.cc
index 4ca1846d45f..ea79b675629 100644
--- a/runtime/vm/object_reload.cc
+++ b/runtime/vm/object_reload.cc
@@ -610,6 +610,11 @@ void Class::MarkFieldBoxedDuringReload(ClassTable* class_table,
 
 bool Class::RequiresInstanceMorphing(ClassTable* class_table,
                                      const Class& replacement) const {
+  if (!is_allocate_finalized()) {
+    // No instances of this class exists on the heap - nothing to morph.
+    return false;
+  }
+
   if (replacement.is_enum_class()) {
     return true;
   }