diff --git a/src/runtime.mjs b/src/runtime.mjs
index 106fc99..903bc73 100644
--- a/src/runtime.mjs
+++ b/src/runtime.mjs
@@ -121,6 +121,9 @@ export async function runScript(source, opts = {}) {
   }
   const out = opts.out || ((s) => console.log(s));
   const max = Number.isFinite(opts.max) ? opts.max : DEFAULT_MAX;
+  if (!Number.isInteger(max) || max < 1) {
+    throw new Error(`moshscript: max must be a positive integer, got ${JSON.stringify(opts.max)}`);
+  }
   const control = makeControl(max, out);
 
   const ctx = {
diff --git a/test/runtime.test.mjs b/test/runtime.test.mjs
index cea89f1..bfadcbc 100644
--- a/test/runtime.test.mjs
+++ b/test/runtime.test.mjs
@@ -112,6 +112,19 @@ test("DEFAULT_MAX bounds an unbounded while when no max is passed", async () =>
   assert.equal(calls.length, DEFAULT_MAX);
 });
 
+test("runScript rejects invalid max values", async () => {
+  const { registry } = recorder();
+
+  await assert.rejects(
+    () => runScript(`while (alive) {}`, { commands: registry, max: 0 }),
+    /max must be a positive integer/,
+  );
+  await assert.rejects(
+    () => runScript(`while (alive) {}`, { commands: registry, max: 1.5 }),
+    /max must be a positive integer/,
+  );
+});
+
 test("a rejecting fire-and-forget verb does not kill a script that keeps running", async () => {
   const calls = [];
   const registry = createRegistry([