diff --git a/test/ts/preact.tsx b/test/ts/preact.tsx
index 68c8fcab3d..3713f9d8f5 100644
--- a/test/ts/preact.tsx
+++ b/test/ts/preact.tsx
@@ -97,7 +97,7 @@ function createRootFragment(parent: Element, replaceNode: Element | Element[]) {
 		parentNode: parent,
 		firstChild: replaceNodes[0],
 		childNodes: replaceNodes,
-		contains: (c) => parent.contains(c),
+		contains: (c: Node) => parent.contains(c),
 		insertBefore: insert,
 		appendChild: (c: Node) => insert(c, null),
 		removeChild: function (c: Node) {