diff --git a/packages/reactivity/src/collectionHandlers.ts b/packages/reactivity/src/collectionHandlers.ts
index af4ca05d..095537cc 100644
--- a/packages/reactivity/src/collectionHandlers.ts
+++ b/packages/reactivity/src/collectionHandlers.ts
@@ -176,8 +176,8 @@ const mutableInstrumentations: Record<string, Function> = {
   get(this: MapTypes, key: unknown) {
     return get(this, key, toReactive)
   },
-  get size(this: IterableCollections) {
-    return size(this)
+  get size() {
+    return size(this as unknown as IterableCollections)
   },
   has,
   add,