Clarify error messages in endpoint routing.

packages/astro/src/runtime/server/endpoint.ts

@@ -23,10 +23,10 @@ export async function renderEndpoint(
 			)} requests are not available for a static site. Update your config to \`output: 'server'\` or \`output: 'hybrid'\` to enable.`
 		);
 	}
-	if (typeof handler !== 'function') {
+	if (handler === undefined) {
 		logger.warn(
 			'router',
-			`No API Route handler exists for the method "${method}" for the route ${url.pathname}.\n` +
+			`No API Route handler exists for the method "${method}" for the route "${url.pathname}".\n` +
 				`Found handlers: ${Object.keys(mod)
 					.map((exp) => JSON.stringify(exp))
 					.join(', ')}\n` +
@@ -38,6 +38,13 @@ export async function renderEndpoint(
 		// 404. Should be handled by 404.astro route if possible.
 		return new Response(null, { status: 404 });
 	}
+	if (typeof handler !== "function") {
+		logger.error(
+			'router',
+			`The route "${url.pathname}" exports a value for the method "${method}", but it is of the type ${typeof handler} instead of a function.`
+		);
+		return new Response(null, { status: 500 });
+	}
 
 	const response = await handler.call(mod, context);
 	// Endpoints explicitly returning 404 or 500 response status should