diff --git a/packages/runtime/src/message-type-contract.ts b/packages/runtime/src/message-type-contract.ts
index ad4d79a1..e9b3587f 100644
--- a/packages/runtime/src/message-type-contract.ts
+++ b/packages/runtime/src/message-type-contract.ts
@@ -62,7 +62,7 @@ export interface IMessageType<T extends object> extends MessageInfo {
      * Contains the prototype for messages returned by create() which
      * includes the `MESSAGE_TYPE` symbol pointing back to `this`.
      */
-    readonly messagePrototype?: Readonly<{}>;
+    readonly messagePrototype?: Readonly<{}> | undefined;
 
 
     /**
diff --git a/packages/runtime/src/message-type.ts b/packages/runtime/src/message-type.ts
index b0c0b203..2c4d58f5 100644
--- a/packages/runtime/src/message-type.ts
+++ b/packages/runtime/src/message-type.ts
@@ -54,7 +54,7 @@ export class MessageType<T extends object> implements IMessageType<T> {
      * Contains the prototype for messages returned by create() which
      * includes the `MESSAGE_TYPE` symbol pointing back to `this`.
      */
-    readonly messagePrototype: Readonly<{}> | undefined;
+    readonly messagePrototype?: Readonly<{}> | undefined;
 
     protected readonly defaultCheckDepth = 16;
     protected readonly refTypeCheck: ReflectionTypeCheck;