diff --git a/demo/web/batch.mjs b/demo/web/batch.mjs
index 5058dbea4f..1899532f98 100644
--- a/demo/web/batch.mjs
+++ b/demo/web/batch.mjs
@@ -6,6 +6,7 @@ function safeSize(value) {
 }
 
 export function memoryEstimate(archiveBytes, selectedBytes, libraryBytes, entries) {
+  for (const value of [archiveBytes, selectedBytes, libraryBytes, entries]) safeSize(value);
   return safeSize(Math.ceil(archiveBytes * 1.02) + selectedBytes * 4 + libraryBytes + entries * 1024 + 8 * 1024 * 1024);
 }
 
diff --git a/demo/web/tests/batch.test.mjs b/demo/web/tests/batch.test.mjs
index b19827d72b..fb774ac7d1 100644
--- a/demo/web/tests/batch.test.mjs
+++ b/demo/web/tests/batch.test.mjs
@@ -2,6 +2,20 @@ import assert from "node:assert/strict";
 import { test } from "node:test";
 import { batchMemoryEstimate, batchOutputDirectory, memoryEstimate, MEMORY_LIMIT } from "../batch.mjs";
 
+test("rejects each invalid memory operand before arithmetic can coerce or cancel it", () => {
+  for (let index = 0; index < 4; index++) {
+    for (const value of [-1, -100000000, "1024", null, true, NaN, Infinity, 1.5, Number.MAX_SAFE_INTEGER + 1]) {
+      const operands = [1024, 70 * 1024 * 1024, 0, 2];
+      operands[index] = value;
+      assert.throws(() => memoryEstimate(...operands), /unsupported file size/u);
+    }
+  }
+  const summaries = [{totalBytes: 1024, selectedBytes: 70 * 1024 * 1024, windowsSelectedBytes: 70 * 1024 * 1024, files: 2}];
+  assert.ok(batchMemoryEstimate(summaries, "linux", 0) > MEMORY_LIMIT);
+  assert.throws(() => batchMemoryEstimate(summaries, "linux", -100000000), /unsupported file size/u);
+  assert.equal(memoryEstimate(0, 0, 0, 0), 8 * 1024 * 1024);
+});
+
 test("estimates all ZIP outputs against one shared memory limit", () => {
   const summaries = [
     { totalBytes: 1024, selectedBytes: 128, windowsSelectedBytes: 256, files: 2 },