diff --git a/packages/rstack/src/rslintConfig.ts b/packages/rstack/src/rslintConfig.ts
index 13dff073..b1f4faa6 100644
--- a/packages/rstack/src/rslintConfig.ts
+++ b/packages/rstack/src/rslintConfig.ts
@@ -2,17 +2,22 @@ import { existsSync } from 'node:fs';
 import { join } from 'node:path';
 import { loadRstackConfig, type LoadedRstackConfig } from './config.ts';
 import type { RslintConfig } from '@rslint/core';
+import { color, logger } from 'rslog';
 
 // Expose the loaded config so `rs check` can pass it to fmt instead of loading
 // and executing the Rstack config a second time.
 export const loadedConfig: LoadedRstackConfig = await loadRstackConfig();
 const { configs } = loadedConfig;
-const lintDefinition = configs.lint ?? [];
+const lintDefinition = configs.lint;
 
 let lintConfig: RslintConfig;
 
-// TODO: support function in Rslint core
-if (typeof lintDefinition === 'function') {
+if (lintDefinition === undefined) {
+  logger.error(
+    `No lint configuration found. Add ${color.cyan('define.lint(...)')} to your Rstack config file.`,
+  );
+  process.exit(1);
+} else if (typeof lintDefinition === 'function') {
   lintConfig = await lintDefinition();
 } else {
   lintConfig = lintDefinition;