diff --git a/package-lock.json b/package-lock.json
index cd7082c2..61a1ad01 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
     "name": "@accordproject/concerto-codegen",
-    "version": "3.20.0",
+    "version": "3.21.0",
     "lockfileVersion": 3,
     "requires": true,
     "packages": {
         "": {
             "name": "@accordproject/concerto-codegen",
-            "version": "3.20.0",
+            "version": "3.21.0",
             "license": "Apache-2.0",
             "dependencies": {
                 "@accordproject/concerto-core": "3.16.4",
diff --git a/package.json b/package.json
index 708ba910..43a6f245 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "@accordproject/concerto-codegen",
-    "version": "3.20.0",
+    "version": "3.21.0",
     "description": "CodeGen for the Concerto Modeling Language",
     "homepage": "https://github.com/accordproject/concerto",
     "engines": {
diff --git a/types/lib/codegen/fromcto/rust/rustvisitor.d.ts b/types/lib/codegen/fromcto/rust/rustvisitor.d.ts
index efddbadc..edf4f7eb 100644
--- a/types/lib/codegen/fromcto/rust/rustvisitor.d.ts
+++ b/types/lib/codegen/fromcto/rust/rustvisitor.d.ts
@@ -55,6 +55,12 @@ declare class RustVisitor {
      * @private
      */
     private visitField;
+    /**
+     * @param {String} type - the type to be wrapped as option
+     * @return {String} - the wrapped type
+     * @private
+     */
+    private wrapAsOption;
     /**
      * Visitor design pattern
      * @param {EnumDeclaration} enumDeclaration - the object being visited