diff --git a/packages/shared/syncRef/index.test.ts b/packages/shared/syncRef/index.test.ts
index 670dce60bfe..48295ff2bf1 100644
--- a/packages/shared/syncRef/index.test.ts
+++ b/packages/shared/syncRef/index.test.ts
@@ -102,6 +102,7 @@ describe('syncRef', () => {
     const ref0 = ref(0)
     const ref1 = ref(1)
     const refString = ref('1')
+    const refNumber = ref(1)
     const refNumString = ref<number | string>(1)
     const refNumBoolean = ref<number | boolean>(1)
     // L = A && direction === 'both'
@@ -253,5 +254,12 @@ describe('syncRef', () => {
     const bool0 = ref(false)
     const bool1 = ref(false)
     syncRef(bool0, bool1)
+
+    syncRef(refNumber, refString, {
+      // @ts-expect-error lack rtl func
+      transform: {
+        ltr: v => String(v),
+      },
+    })
   })
 })
diff --git a/packages/shared/syncRef/index.ts b/packages/shared/syncRef/index.ts
index 544fb5bf88c..de06acd120c 100644
--- a/packages/shared/syncRef/index.ts
+++ b/packages/shared/syncRef/index.ts
@@ -130,7 +130,7 @@ export type SyncRefOptions<L, R, D extends Direction> = ConfigurableFlushSync &
  * 3. L ⊆ R
  * 4. L ∩ R = ∅
  */
-export function syncRef<L, R, D extends Direction>(
+export function syncRef<L, R, D extends Direction = 'both'>(
   left: Ref<L>,
   right: Ref<R>,
   ...[options]: Equal<L, R> extends true