diff --git a/deno/utils/internals.ts b/deno/utils/internals.ts
index 21510966..0ec17340 100644
--- a/deno/utils/internals.ts
+++ b/deno/utils/internals.ts
@@ -7,7 +7,9 @@ export type Nullable<T> = {
  * (since JSON.stringify ignores undefined properties)
  */
 export type AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Base> = {
-	[K in keyof Base]: Base[K] extends Exclude<Base[K], undefined> ? Base[K] : Base[K] | undefined;
+	[K in keyof Base]: Base[K] extends Exclude<Base[K], undefined>
+		? AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Base[K]>
+		: AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Base[K]> | undefined;
 };
 
 export type StrictPartial<Base> = AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Partial<Base>>;
diff --git a/utils/internals.ts b/utils/internals.ts
index 21510966..0ec17340 100644
--- a/utils/internals.ts
+++ b/utils/internals.ts
@@ -7,7 +7,9 @@ export type Nullable<T> = {
  * (since JSON.stringify ignores undefined properties)
  */
 export type AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Base> = {
-	[K in keyof Base]: Base[K] extends Exclude<Base[K], undefined> ? Base[K] : Base[K] | undefined;
+	[K in keyof Base]: Base[K] extends Exclude<Base[K], undefined>
+		? AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Base[K]>
+		: AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Base[K]> | undefined;
 };
 
 export type StrictPartial<Base> = AddUndefinedToPossiblyUndefinedPropertiesOfInterface<Partial<Base>>;