diff --git a/bin/blockclock.js b/bin/blockclock.js
index 49b7592..66a25e3 100644
--- a/bin/blockclock.js
+++ b/bin/blockclock.js
@@ -85,7 +85,9 @@ function parseArgs(argv) {
       continue;
     }
     const value = argv[++i];
-    if (value === undefined) throw new UsageError(`--${key} needs a value`);
+    if (value === undefined || value.startsWith('--')) {
+      throw new UsageError(`--${key} needs a value`);
+    }
     out[key] = value;
   }
   return out;
diff --git a/test/cli-arguments.test.js b/test/cli-arguments.test.js
new file mode 100644
index 0000000..8dfb785
--- /dev/null
+++ b/test/cli-arguments.test.js
@@ -0,0 +1,51 @@
+// SPDX-FileCopyrightText: 2026 1932819331hy-debug
+// SPDX-License-Identifier: MIT
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+
+const cli = fileURLToPath(new URL('../bin/blockclock.js', import.meta.url));
+function run(args) {
+  const result = spawnSync(process.execPath, [cli, ...args], {
+    encoding: 'utf8', timeout: 10000,
+  });
+  assert.ifError(result.error);
+  assert.equal(result.signal, null);
+  return result;
+}
+
+// An option token is not the missing value of the preceding option.
+// In particular, treating --json as an RPC URL misclassifies usage as exit 3.
+const schedule = ['--start-block', '0', '--period-blocks', '100', '--periods', '20',
+  '--design-block-time', '2'];
+for (const next of ['--json', '--help', '--unknown']) {
+  test(`a missing --rpc value before ${next} is a usage error`, () => {
+    const r = run([...schedule, '--rpc', next]);
+    assert.equal(r.status, 2, r.stderr);
+    assert.equal(r.stdout, '');
+    assert.match(r.stderr, /--rpc needs a value/);
+    assert.doesNotMatch(r.stderr, /attempts|Failed to parse URL/);
+  });
+}
+
+test('a missing value at the end is still a usage error', () => {
+  const r = run([...schedule, '--rpc']);
+  assert.equal(r.status, 2);
+  assert.match(r.stderr, /--rpc needs a value/);
+});
+
+test('a negative numeric value is still diagnosed as a numeric argument', () => {
+  const r = run(['--chain', 'bsc', ...schedule, '--periods', '-1']);
+  assert.equal(r.status, 2);
+  assert.match(r.stderr, /--periods must be greater than 0/);
+  assert.doesNotMatch(r.stderr, /needs a value/);
+});
+
+test('--help still succeeds without schedule arguments', () => {
+  const r = run(['--help']);
+  assert.equal(r.status, 0);
+  assert.match(r.stdout, /Usage:/);
+  assert.equal(r.stderr, '');
+});