diff --git a/tunnel-server/src/tunnel-store/array-map.test.ts b/tunnel-server/src/tunnel-store/array-map.test.ts
index 25e06f18..fd4f8af2 100644
--- a/tunnel-server/src/tunnel-store/array-map.test.ts
+++ b/tunnel-server/src/tunnel-store/array-map.test.ts
@@ -17,7 +17,7 @@ describe('arrayMap', () => {
   })
 
   describe('when the key exists', () => {
-    let values: { x: number }[] | undefined
+    let values: readonly { x: number }[] | undefined
     beforeEach(() => {
       values = a.get('foo')
     })