diff --git a/package.json b/package.json
index b61275e..2080619 100644
--- a/package.json
+++ b/package.json
@@ -6,8 +6,8 @@
     "license": "GPL-3.0",
     "private": true,
     "scripts": {
-        "test": "jest --maxWorkers=3",
-        "test:verbose": "jest --maxWorkers=3 --verbose",
+        "test": "node script/lib/jest.js --maxWorkers=3",
+        "test:verbose": "node script/lib/jest.js --maxWorkers=3 --verbose",
         "build": "node build.js",
         "lint": "eslint . && prettier --check .",
         "fmt": "prettier --write .",
diff --git a/script/lib/jest.js b/script/lib/jest.js
new file mode 100644
index 0000000..1aa0511
--- /dev/null
+++ b/script/lib/jest.js
@@ -0,0 +1,33 @@
+// Runs jest with the arguments it is given, and fails the run when jest had to
+// force-stop a worker. Jest itself only prints a warning when that happens, and
+// its run still passes. The warning means a test left a timer or another open
+// handle running. The test scripts in package.json run jest through this file.
+
+"use strict";
+
+const { spawn } = require("child_process");
+
+// The start of the warning that jest prints on stderr after it force-stops a
+// worker.
+const WORKER_WARNING = "A worker process has failed to exit gracefully";
+
+const jest = spawn("jest", process.argv.slice(2), {
+    stdio: ["inherit", "inherit", "pipe"],
+});
+
+let stderr = "";
+jest.stderr.on("data", (chunk) => {
+    process.stderr.write(chunk);
+    stderr += chunk;
+});
+
+jest.on("close", (code) => {
+    if (stderr.includes(WORKER_WARNING)) {
+        console.error(
+            "script/lib/jest.js: failing the run, because jest force-stopped " +
+                "a worker. A test left a timer or another open handle running.",
+        );
+        process.exit(1);
+    }
+    process.exit(code ?? 1);
+});