diff --git a/lib/index.js b/lib/index.js
index edd8988..aa7b55d 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -140,6 +140,10 @@ const open = (_args, opts = {}, extra = {}) => {
   // RC file, e.g. .bashrc or .zshrc.
   if (platform === 'linux' && os.release().toLowerCase().includes('microsoft')) {
     platform = 'wsl'
+    if (!process.env.BROWSER) {
+      return Promise.reject(
+        new Error('Set the BROWSER environment variable to your desired browser.'))
+    }
   }
 
   let command = options.command
diff --git a/test/open.js b/test/open.js
index 3918749..a5fa2ae 100644
--- a/test/open.js
+++ b/test/open.js
@@ -159,12 +159,14 @@ t.test('process.platform === linux', (t) => {
     t.ok(proc.called)
   })
 
-  t.test('when os.release() includes Microsoft treats as win32', async (t) => {
+  t.test('when os.release() includes Microsoft treats as WSL', async (t) => {
     const promiseSpawnMock = t.mock('../lib/index.js', {
       os: {
         release: () => 'Microsoft',
       },
     })
+    const browser = process.env.BROWSER
+    process.env.BROWSER = '/mnt/c/Program Files (x86)/Google/Chrome/Application/chrome.exe'
 
     const proc = spawk.spawn('sh', ['-c', 'sensible-browser https://google.com'], { shell: false })
 
@@ -174,15 +176,21 @@ t.test('process.platform === linux', (t) => {
       signal: undefined,
     })
 
+    t.teardown(() => {
+      process.env.BROWSER = browser
+    })
+
     t.ok(proc.called)
   })
 
-  t.test('when os.release() includes microsoft treats as win32', async (t) => {
+  t.test('when os.release() includes microsoft treats as WSL', async (t) => {
     const promiseSpawnMock = t.mock('../lib/index.js', {
       os: {
         release: () => 'microsoft',
       },
     })
+    const browser = process.env.BROWSER
+    process.env.BROWSER = '/mnt/c/Program Files (x86)/Google/Chrome/Application/chrome.exe'
 
     const proc = spawk.spawn('sh', ['-c', 'sensible-browser https://google.com'], { shell: false })
 
@@ -192,9 +200,35 @@ t.test('process.platform === linux', (t) => {
       signal: undefined,
     })
 
+    t.teardown(() => {
+      process.env.BROWSER = browser
+    })
+
     t.ok(proc.called)
   })
 
+  t.test('fails on WSL if BROWSER is not set', async (t) => {
+    const promiseSpawnMock = t.mock('../lib/index.js', {
+      os: {
+        release: () => 'microsoft',
+      },
+    })
+    const browser = process.env.BROWSER
+    delete process.env.BROWSER
+
+    const proc = spawk.spawn('sh', ['-c', 'sensible-browser https://google.com'], { shell: false })
+
+    await t.rejects(promiseSpawnMock.open('https://google.com'), {
+      message: 'Set the BROWSER environment variable to your desired browser.',
+    })
+
+    t.teardown(() => {
+      process.env.BROWSER = browser
+    })
+
+    t.notOk(proc.called)
+  })
+
   t.end()
 })