diff --git a/src/commands.mjs b/src/commands.mjs
index 7cacf74..e5c2211 100644
--- a/src/commands.mjs
+++ b/src/commands.mjs
@@ -65,7 +65,11 @@ export function defaultCommands() {
     // handy extras
     say: (ctx, args) => ctx.out(`  💬 ${args.join(" ")}`),
     sleep: async (_ctx, args) => {
-      const ms = Number(args[0] || 0);
+      const raw = args[0] ?? 0;
+      const ms = Number(raw);
+      if (!Number.isFinite(ms) || ms < 0) {
+        throw new Error(`moshscript: sleep(ms) requires a finite non-negative number, got ${JSON.stringify(raw)}`);
+      }
       if (ms > 0) await new Promise((r) => setTimeout(r, ms));
     },
     stop: (ctx, args) => {
diff --git a/test/commands.test.mjs b/test/commands.test.mjs
index 0ad9f08..79d4fdf 100644
--- a/test/commands.test.mjs
+++ b/test/commands.test.mjs
@@ -34,3 +34,15 @@ test("notify() still accepts message arguments", async () => {
   assert.equal(ctx.lines.length, 1);
   assert.match(ctx.lines[0], /hello there/);
 });
+
+test("sleep accepts zero milliseconds", async () => {
+  await defaultCommands().sleep({}, [0]);
+});
+
+test("sleep rejects non-finite and negative durations", async () => {
+  const sleep = defaultCommands().sleep;
+
+  await assert.rejects(() => sleep({}, ["forever"]), /finite non-negative number/);
+  await assert.rejects(() => sleep({}, ["Infinity"]), /finite non-negative number/);
+  await assert.rejects(() => sleep({}, [-1]), /finite non-negative number/);
+});