diff --git a/tests/public-api.test.ts b/tests/public-api.test.ts
index 91f1fda..06fa420 100644
--- a/tests/public-api.test.ts
+++ b/tests/public-api.test.ts
@@ -4,6 +4,7 @@ import {
     ClassNameValue,
     ClassValidator,
     Config,
+    ConfigExtension,
     DefaultClassGroupIds,
     DefaultThemeGroupIds,
     createTailwindMerge,
@@ -183,7 +184,9 @@ test('mergeConfigs has correct inputs and outputs', () => {
 })
 
 test('extendTailwindMerge has correct inputs and outputs', () => {
-    expect(extendTailwindMerge({})).toStrictEqual(expect.any(Function))
+    expect(extendTailwindMerge({} satisfies ConfigExtension<string, string>)).toStrictEqual(
+        expect.any(Function),
+    )
 })
 
 test('fromTheme has correct inputs and outputs', () => {