diff --git a/src/duration.test.ts b/src/duration.test.ts
index 02782cf..8a06f0a 100644
--- a/src/duration.test.ts
+++ b/src/duration.test.ts
@@ -8,4 +8,29 @@ describe('parseDuration', () => {
     expect(parseDuration('5m')).toBe(300_000);
     expect(parseDuration('2h')).toBe(7_200_000);
   });
+
+  it('accepts zero', () => {
+    expect(parseDuration('0s')).toBe(0);
+  });
+
+  it('ignores surrounding whitespace', () => {
+    expect(parseDuration(' 5m ')).toBe(300_000);
+    expect(parseDuration('\t30s\n')).toBe(30_000);
+  });
+
+  it.each([
+    ['5x'],
+    [''],
+    ['   '],
+    ['5'],
+    ['m'],
+    ['-5s'],
+    ['1.5s'],
+    ['5 m'],
+    ['5M'],
+    ['5mm'],
+  ])('throws RangeError naming %j', (bad) => {
+    expect(() => parseDuration(bad)).toThrow(RangeError);
+    expect(() => parseDuration(bad)).toThrow(JSON.stringify(bad));
+  });
 });
diff --git a/src/duration.ts b/src/duration.ts
index b1fd194..5f34383 100644
--- a/src/duration.ts
+++ b/src/duration.ts
@@ -7,6 +7,11 @@ const UNIT_MS: Record<string, number> = {
 
 /** Parse "250ms", "30s", "5m", "2h" into milliseconds. */
 export function parseDuration(input: string): number {
-  const match = /^(\d+)(ms|s|m|h)$/.exec(input);
-  return Number(match![1]) * UNIT_MS[match![2]];
+  const match = /^(\d+)(ms|s|m|h)$/.exec(input.trim());
+  if (!match) {
+    throw new RangeError(
+      `invalid duration ${JSON.stringify(input)}: expected an integer followed by ms, s, m or h`,
+    );
+  }
+  return Number(match[1]) * UNIT_MS[match[2]];
 }