diff --git a/compiler/cpp/src/thrift.l b/compiler/cpp/src/thrift.l
index b7008c6..80cbf87 100644
--- a/compiler/cpp/src/thrift.l
+++ b/compiler/cpp/src/thrift.l
@@ -60,6 +60,7 @@
 "java_package"  { return tok_java_package;  }
 "php_namespace" { return tok_php_namespace; }
 "xsd_all"       { return tok_xsd_all;       }
+"xsd_optional"  { return tok_xsd_optional;  }
 "include"       { return tok_include;       }
 
 "void"          { return tok_void;          }