diff --git a/src/billing.ts b/src/billing.ts
index 692af1d..630a9f7 100644
--- a/src/billing.ts
+++ b/src/billing.ts
@@ -1,3 +1,3 @@
 export function charge(amount: number): string {
-  return `charged ${amount}`;
+  throw new Error("not implemented: charge");
 }