diff --git a/compiler/cpp/src/thrift/generate/t_erl_generator.cc b/compiler/cpp/src/thrift/generate/t_erl_generator.cc
index 372c78b..9e1e23d 100644
--- a/compiler/cpp/src/thrift/generate/t_erl_generator.cc
+++ b/compiler/cpp/src/thrift/generate/t_erl_generator.cc
@@ -810,6 +810,8 @@
   if (has_default_value(tmember))
     out << " = " << render_member_value(tmember);
   out << " :: " << render_member_type(tmember);
+  if (tmember->get_req() != t_field::T_REQUIRED)
+    out << " | 'undefined'";
 }
 
 bool t_erl_generator::has_default_value(t_field* field) {