diff --git a/handler.ts b/handler.ts
index 0a8335e..15cc4b3 100644
--- a/handler.ts
+++ b/handler.ts
@@ -8,7 +8,7 @@ function setXResponseTimeHeader<C extends Context>(ctx: C, startTime: number) {
   ctx.response.headers.set("X-Response-Time", `${ms}ms`);
 }
 
-function assertError(caught: unknown): Error {
+export function assertError(caught: unknown): Error {
   return caught instanceof Error ? caught : new Error("[non-error thrown]");
 }
 
diff --git a/mod.ts b/mod.ts
index e10df20..caa9e1f 100644
--- a/mod.ts
+++ b/mod.ts
@@ -1,5 +1,5 @@
 export { listen } from "./server.ts";
 export { Context } from "./context.ts";
 export * from "./route.ts";
-export { createHandler, type Handler } from "./handler.ts";
+export { assertError, createHandler, type Handler } from "./handler.ts";
 export { compose, composeSync } from "./composition.ts";