diff --git a/packages/runtime/src/message-type-contract.ts b/packages/runtime/src/message-type-contract.ts
index d4590101..b305fbf9 100644
--- a/packages/runtime/src/message-type-contract.ts
+++ b/packages/runtime/src/message-type-contract.ts
@@ -22,7 +22,7 @@ export type PartialMessage<T extends object> = {
      T extends (Date | Uint8Array | bigint | boolean | string | number) ? T
    : T extends Array<infer U> ? Array<PartialField<U>>
    : T extends ReadonlyArray<infer U> ? ReadonlyArray<PartialField<U>>
-   : T extends { oneofKind: string } ? T
+   : T extends { oneofKind: string & keyof T } ? { [k in T['oneofKind']]: { oneofKind: k } & { [p in k]: PartialField<T[k]> } }[T['oneofKind']]
    : T extends { oneofKind: undefined } ? T
    : T extends object ? PartialMessage<T>
    : T ;