diff --git a/compiler/cpp/src/thrifty.yy b/compiler/cpp/src/thrifty.yy
index bf5408e..2c16ca0 100644
--- a/compiler/cpp/src/thrifty.yy
+++ b/compiler/cpp/src/thrifty.yy
@@ -830,10 +830,11 @@
     {
       pdebug("FieldList -> FieldList , Field");
       $$ = $1;
-      if (!($$->append($2))) {
+      if (!($$->validate_field($2))) {
         yyerror("Field identifier %d for \"%s\" has already been used", $2->get_key(), $2->get_name().c_str());
         exit(1);
       }
+      $$->append($2);
     }
 |
     {