diff --git a/test/isNil.test.ts b/test/isNil.test.ts
new file mode 100644
index 0000000..d463890
--- /dev/null
+++ b/test/isNil.test.ts
@@ -0,0 +1,26 @@
+import { expectType } from 'tsd';
+import { filter, isNil } from '../es';
+
+expectType<boolean>(isNil(1));
+expectType<boolean>(isNil('a'));
+expectType<boolean>(isNil(true));
+
+expectType<boolean>(isNil(null));
+expectType<boolean>(isNil(undefined));
+
+const maybeNumber = (): number | null => {
+  if (Math.random() > 0.5) {
+    return null;
+  }
+  return 123;
+};
+
+expectType<number | null>(maybeNumber());
+
+const r = maybeNumber();
+
+if (isNil(r)) {
+  expectType<null>(r);
+}
+
+expectType<undefined[]>(filter(isNil, [] as (string | undefined)[]));
diff --git a/test/isNotNil.test.ts b/test/isNotNil.test.ts
index 61049da..5f67478 100644
--- a/test/isNotNil.test.ts
+++ b/test/isNotNil.test.ts
@@ -1,5 +1,5 @@
 import { expectType } from 'tsd';
-import { isNotNil } from '../es';
+import { filter, isNotNil } from '../es';
 
 expectType<boolean>(isNotNil(1));
 expectType<boolean>(isNotNil('a'));
@@ -22,3 +22,5 @@ const r = maybeNumber();
 if (isNotNil(r)) {
   expectType<number>(r);
 }
+
+expectType<string[]>(filter(isNotNil, [] as (string | undefined)[]));
diff --git a/types/isNil.d.ts b/types/isNil.d.ts
index 2d1d2e5..d3c51d7 100644
--- a/types/isNil.d.ts
+++ b/types/isNil.d.ts
@@ -1 +1 @@
-export function isNil(value: any): value is null | undefined;
+export function isNil<T>(value: T): value is ((null | undefined) & T);