diff --git a/__tests__/utils/configSweep.test.ts b/__tests__/utils/configSweep.test.ts
new file mode 100644
index 00000000..e3f66d81
--- /dev/null
+++ b/__tests__/utils/configSweep.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it, vi } from 'vitest'
+
+import { parseProwConfig } from '../../src/utils/config'
+import * as duration from '../../src/utils/duration'
+
+vi.mock('../../src/utils/duration', { spy: true })
+
+describe('normalizeSweep', () => {
+  it('accepts an empty sweep mapping with no lookback', () => {
+    expect(parseProwConfig('x', 'sweep: {}\n')).toEqual({ sweep: {} })
+  })
+
+  it('stringifies a non-Error thrown while parsing sweep.lookback', () => {
+    vi.mocked(duration.parseDuration).mockImplementationOnce(() => {
+      throw 'clock skew' // eslint-disable-line no-throw-literal
+    })
+
+    expect(() => parseProwConfig('x', 'sweep:\n  lookback: 1h\n')).toThrow('x: clock skew')
+  })
+})