diff --git a/packages/proto-loader/bin/proto-loader-gen-types.ts b/packages/proto-loader/bin/proto-loader-gen-types.ts
index b586983d..bdd7d789 100644
--- a/packages/proto-loader/bin/proto-loader-gen-types.ts
+++ b/packages/proto-loader/bin/proto-loader-gen-types.ts
@@ -398,7 +398,7 @@ function generateRestrictedMessageInterface(formatter: TextFormatter, messageTyp
       if (options.includeComments) {
         formatComment(formatter, oneof.comment, oneof.options);
       }
-      formatter.writeLine(`'${oneof.name}': ${typeString};`);
+      formatter.writeLine(`'${oneof.name}'?: ${typeString};`);
     }
   }
   if (options.outputBranded) {