diff --git a/index.d.ts b/index.d.ts
index a5ae519..c65aa30 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,2 +1,2 @@
-declare function isEqual<B>(a: any, b: B): a is B;
+declare function isEqual<A = any, B = any>(a: A, b: B): boolean;
 export = isEqual;
diff --git a/test/typescript/sample-usage.tsx b/test/typescript/sample-usage.tsx
index d9c7b91..0bd46e1 100644
--- a/test/typescript/sample-usage.tsx
+++ b/test/typescript/sample-usage.tsx
@@ -31,6 +31,14 @@ class TestChild extends React.Component<IProps> {
   }
 }
 
+// `equal` must return a plain `boolean`, not a type guard (`a is B`): after a
+// `false` result, a guard narrows `prev` to `never`, so `prev.text` would not
+// compile (#130).
+export function changedText(prev: ITodo, next: ITodo): string | null {
+  if (equal(prev, next)) return null;
+  return prev.text;
+}
+
 class TestContainer extends React.Component {
   render() {
     return testArr.map((item) => <TestChild key={item.id} todo={item} />);