diff --git a/test/builtins.test.ts b/test/builtins.test.ts
index e4183e2..5b16ef3 100644
--- a/test/builtins.test.ts
+++ b/test/builtins.test.ts
@@ -20,7 +20,7 @@ import {
 } from "~";
 
 const isEven = (n: number) => n % 2 === 0;
-const isEvenIndex = (_, index: number) => index % 2 === 0;
+const isEvenIndex = (_: unknown, index: number) => index % 2 === 0;
 
 function isNum(value: unknown): value is number {
   return typeof value === "number";
diff --git a/test/itertools.test.ts b/test/itertools.test.ts
index 6d52c35..44de48b 100644
--- a/test/itertools.test.ts
+++ b/test/itertools.test.ts
@@ -21,7 +21,7 @@ import {
 } from "~";
 
 const isEven = (x: number) => x % 2 === 0;
-const isEvenIndex = (_, index: number) => index % 2 === 0;
+const isEvenIndex = (_: unknown, index: number) => index % 2 === 0;
 const isPositive = (x: number) => x >= 0;
 
 function isNum(value: unknown): value is number {
diff --git a/test/more-itertools.test.ts b/test/more-itertools.test.ts
index 59ebd8a..c044e46 100644
--- a/test/more-itertools.test.ts
+++ b/test/more-itertools.test.ts
@@ -17,7 +17,7 @@ import {
 import * as fc from "fast-check";
 
 const isEven = (x: number) => x % 2 === 0;
-const isEvenIndex = (_, index: number) => index % 2 === 0;
+const isEvenIndex = (_: unknown, index: number) => index % 2 === 0;
 const isPositive = (x: number) => x >= 0;
 
 function isNum(value: unknown): value is number {