diff --git a/index.js b/index.js
index ec51a29..f009010 100644
--- a/index.js
+++ b/index.js
@@ -83,11 +83,27 @@ function determineBuildId (id, inputDir) {
   if (id) return Promise.resolve(id)
   return new Promise((resolve, reject) => {
     const cp = require('child_process')
-    cp.execFile('git', [`--git-dir=${inputDir}/.git`, `--work-tree=${inputDir}`, 'rev-parse', 'HEAD'], (err, stdout, stderr) => {
+
+    // inputDir may not be the project root so look for .git dir in parent dirs too
+    let dir = inputDir
+    const root = path.parse(dir).root
+    let attempts = 0 // protect against infinite tight loop if libs misbehave
+    while (dir !== root && attempts < 999) {
+      attempts++
+      try {
+        fs.accessSync(path.join(dir, '.git'), (fs.constants || fs).R_OK)
+        break
+      } catch (_) {
+        dir = path.dirname(dir)
+      }
+    }
+    if (dir === root || attempts >= 999) dir = inputDir
+
+    cp.execFile('git', [`--git-dir=${path.join(dir, '.git')}`, `--work-tree=${dir}`, 'rev-parse', 'HEAD'], (err, stdout, stderr) => {
       if (err) return reject(err)
       if (stderr) return reject(String(stderr).trim())
       if (stdout) return resolve(String(stdout).trim())
-      reject(`No output from command: git --git-dir=${inputDir}/.git --work-tree=${inputDir} rev-parse HEAD`)
+      reject(`No output from command: git --git-dir=${path.join(dir, '.git')} --work-tree=${dir} rev-parse HEAD`)
     })
   })
 }
diff --git a/test/git b/test/git
index b8a1fa1..61e90b6 100755
--- a/test/git
+++ b/test/git
@@ -1,4 +1,12 @@
 #!/usr/bin/env node
-if (!String(process.argv[3]).includes('fixture3')) {
-  console.log('0123456789abcdef0123456789abcdef01234567')
-}
+
+// --git-dir=${inputDir}/.git --work-tree=${inputDir}
+require('sywac')
+  .dir('--git-dir <dir>', { mustExist: true })
+  .outputSettings({ showHelpOnError: false })
+  .parseAndExit()
+  .then(argv => {
+    if (!process.env.NBI_TEST_FIXTURE || !String(process.env.NBI_TEST_FIXTURE).includes('fixture3')) {
+      console.log('0123456789abcdef0123456789abcdef01234567')
+    }
+  })
diff --git a/test/test-fixture.js b/test/test-fixture.js
index 6145f87..27ab5d4 100644
--- a/test/test-fixture.js
+++ b/test/test-fixture.js
@@ -88,7 +88,7 @@ tap.test('supports custom --id', t => {
 
 tap.test('does not need git with custom --id', t => {
   return exec('npm', 'run build', fixturePath).then(() => {
-    return cli('--id 123456', fixturePath, '/dne')
+    return cli('--id 123456', fixturePath, { PATH: '/dne' })
   }).then(io => {
     t.notOk(io.err)
     t.notOk(io.stderr)
diff --git a/test/test-others.js b/test/test-others.js
index 368bbcb..ee61606 100644
--- a/test/test-others.js
+++ b/test/test-others.js
@@ -21,7 +21,7 @@ tap.test('errs on invalid json', t => {
 })
 
 tap.test('errs on no git output', t => {
-  return cli('', path.resolve(__dirname, 'fixture3')).then(io => {
+  return cli('', path.resolve(__dirname, 'fixture3'), { NBI_TEST_FIXTURE: 'fixture3' }).then(io => {
     t.equal(io.err.code, 1)
     t.match(io.stderr, /No output from command: git/)
     t.notOk(io.stdout)
@@ -29,7 +29,7 @@ tap.test('errs on no git output', t => {
 })
 
 tap.test('errs without --id and without git', t => {
-  return cli('', path.resolve(__dirname, 'fixture3'), '/dne').then(io => {
+  return cli('', path.resolve(__dirname, 'fixture3'), { NBI_TEST_FIXTURE: 'fixture3', PATH: '/dne' }).then(io => {
     t.equal(io.err.code, 1)
     t.match(io.stderr, /Unexpected error/)
     t.notOk(io.stdout)
diff --git a/test/utils.js b/test/utils.js
index bb5453d..8addc82 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -21,14 +21,14 @@ function exec (file, args, cwd, env, alwaysResolve) {
   })
 }
 
-function mockedGitEnv (envPath) {
-  const env = Object.assign({}, process.env)
-  env.PATH = envPath || [__dirname].concat(env.PATH.split(path.delimiter)).join(path.delimiter)
-  return env
+function mockedGitEnv (env) {
+  env = env || {}
+  if (!env.PATH) env.PATH = [__dirname].concat(process.env.PATH.split(path.delimiter)).join(path.delimiter)
+  return Object.assign({}, process.env, env)
 }
 
-function cli (args, cwd, envPath) {
-  return exec(cliPath, args, cwd, mockedGitEnv(envPath), true)
+function cli (args, cwd, env) {
+  return exec(cliPath, args, cwd, mockedGitEnv(env), true)
 }
 
 function readTextFile (file) {