diff --git a/index.js b/index.js
index 001a90b3..6d32f27d 100644
--- a/index.js
+++ b/index.js
@@ -59,6 +59,8 @@ class Updates {
     } else if (platform !== 'darwin' && platform !== 'win32') {
       const message = `Unsupported platform: "${platform}". Supported: darwin, win32.`
       notFound(res, message)
+    } else if (version && !semver.valid(version)) {
+      badRequest(res, `Invalid SemVer: "${version}"`)
     } else if (file === 'RELEASES') {
       await this.handleReleases(res, account, repository)
     } else {
@@ -203,6 +205,11 @@ const notFound = (res, message = 'Not found') => {
   res.end(message)
 }
 
+const badRequest = (res, message) => {
+  res.statusCode = 400
+  res.end(message)
+}
+
 const noContent = res => {
   res.statusCode = 204
   res.end()
diff --git a/test/index.js b/test/index.js
index 107f9b09..87cf1327 100644
--- a/test/index.js
+++ b/test/index.js
@@ -181,6 +181,13 @@ test('Updates', async t => {
       }
     })
 
+    await t.test('invalid semver', async t => {
+      const res = await fetch(`${address}/owner/repo/darwin/latest`)
+      t.equal(res.status, 400)
+      const body = await res.text()
+      t.equal(body, 'Invalid SemVer: "latest"')
+    })
+
     await t.test('exists but has no releases', async t => {
       for (let i = 0; i < 2; i++) {
         const res = await fetch(