Don't use syntax()
diff --git a/src/mutator.cc b/src/mutator.cc
index fb5542b..78ef455 100644
--- a/src/mutator.cc
+++ b/src/mutator.cc
@@ -87,11 +87,7 @@
 }
 
 bool IsProto3SimpleField(const FieldDescriptor& field) {
-  assert(field.file()->syntax() == FileDescriptor::SYNTAX_PROTO3 ||
-         field.file()->syntax() == FileDescriptor::SYNTAX_PROTO2);
-  return field.file()->syntax() == FileDescriptor::SYNTAX_PROTO3 &&
-         field.cpp_type() != FieldDescriptor::CPPTYPE_MESSAGE &&
-         !field.containing_oneof() && !field.is_repeated();
+  return !field.is_repeated() && !field.has_presence();
 }
 
 struct CreateDefaultField : public FieldFunction<CreateDefaultField> {