diff --git a/commit-stream.js b/commit-stream.js
index 8b7c07e..9ae2d5f 100644
--- a/commit-stream.js
+++ b/commit-stream.js
@@ -46,7 +46,13 @@ export default function commitStream (ghUser, ghProject) {
   return new Transform({
     objectMode: true,
     transform (line, _, callback) {
-      emit(this, addLine(line))
+      let done
+      try {
+        done = addLine(line)
+      } catch (err) {
+        return callback(/** @type {Error} */ (err))
+      }
+      emit(this, done)
       callback()
     },
     flush (callback) {
@@ -73,7 +79,7 @@ export default function commitStream (ghUser, ghProject) {
     let m
 
     if (!commit) {
-      throw new Error('Something unexpected occurred')
+      throw new Error(`Unexpected line before first "commit" header: ${line}`)
     }
 
     if ((m = line.match(/^CommitDate:\s+(.*)$/)) !== null) {
diff --git a/test.js b/test.js
index 0ffa43f..04cfd69 100644
--- a/test.js
+++ b/test.js
@@ -233,3 +233,17 @@ test('cve id with 4 or more sequence digits', async () => {
   const commits = await toCommits(ids.flatMap(log))
   assert.deepStrictEqual(commits.map((c) => c.cveId), ids)
 })
+
+test('line before first commit header rejects pipeline', async () => {
+  await assert.rejects(toCommits(['garbage']), {
+    message: 'Unexpected line before first "commit" header: garbage'
+  })
+})
+
+test('line before first commit header emits error on write', async () => {
+  const stream = commitStream()
+  const error = once(stream, 'error')
+  stream.write('garbage')
+  const [err] = await error
+  assert.strictEqual(err.message, 'Unexpected line before first "commit" header: garbage')
+})
diff --git a/types/commit-stream.d.ts.map b/types/commit-stream.d.ts.map
index f5cbe0b..b670c7e 100644
--- a/types/commit-stream.d.ts.map
+++ b/types/commit-stream.d.ts.map
@@ -1 +1 @@
-{"version":3,"file":"commit-stream.d.ts","sourceRoot":"","sources":["../commit-stream.js"],"names":[],"mappings":"AAAA,OAAO,EAAE,SAAS,EAAE,MAAM,aAAa,CAAA;AAIpC,YAAkB,MAAM,GACxB;IAAmB,IAAI,EAAZ,MAAM,CACjB;IAAmB,KAAK,EAAb,MAAM,CACnB;CAAA,CAAA;AAGE,YAAkB,OAAO,GACzB;IAAmB,KAAK,EAAb,MAAM,CACjB;IAAmB,UAAU,EAAlB,MAAM,CACjB;IAAmB,SAAS,EAAjB,MAAM,CACnB;CAAA,CAAA;AAGE,YAAkB,MAAM,GACxB;IAAmB,GAAG,EAAX,MAAM,CACjB;IAAoB,MAAM,AAA1B,CACA,EADW,MAAM,CACjB;IAAsB,OAAO,AAA7B,CACA,EADW,MAAM,EAAE,CACnB;IAAoB,UAAU,AAA9B,CACA,EADW,MAAM,CACjB;IAAoB,UAAU,AAA9B,CACA,EADW,MAAM,CACjB;IAAoB,OAAO,AAA3B,CACA,EADW,MAAM,CACjB;IAAsB,WAAW,AAAjC,CACA,EADW,MAAM,EAAE,CACnB;IAAsB,SAAS,AAA/B,CACA,EADW,MAAM,EAAE,CACnB;IAAqB,OAAO,AAA5B,CACA,EADW,OAAO,CAClB;IAAoB,KAAK,AAAzB,CACA,EADW,MAAM,CACjB;IAAoB,MAAM,AAA1B,CACA,EADW,MAAM,CACjB;IAAoB,SAAS,AAA7B,CACA,EADW,MAAM,CACjB;IAAoB,OAAO,AAA3B,CACA,EADW,MAAM,CACjB;IAAoB,KAAK,AAAzB,CAA0B,EAAf,MAAM,CAAS;CAAA,CAAA;AA5B7B;;;;GAIG;AAEH;;;;;GAKG;AAEH;;;;;;;;;;;;;;;;GAgBG;AAEH;;;;;;GAMG;AACH,MAAM,CAAC,OAAO,UAAU,YAAY,CAAE,MAAM,AAJzC,CACA,EADQ,MAIiC,EAAE,SAAS,AAHpD,CACA,EADQ,MAG4C,GAF1C,SAAS,CA6FrB"}
\ No newline at end of file
+{"version":3,"file":"commit-stream.d.ts","sourceRoot":"","sources":["../commit-stream.js"],"names":[],"mappings":"AAAA,OAAO,EAAE,SAAS,EAAE,MAAM,aAAa,CAAA;AAIpC,YAAkB,MAAM,GACxB;IAAmB,IAAI,EAAZ,MAAM,CACjB;IAAmB,KAAK,EAAb,MAAM,CACnB;CAAA,CAAA;AAGE,YAAkB,OAAO,GACzB;IAAmB,KAAK,EAAb,MAAM,CACjB;IAAmB,UAAU,EAAlB,MAAM,CACjB;IAAmB,SAAS,EAAjB,MAAM,CACnB;CAAA,CAAA;AAGE,YAAkB,MAAM,GACxB;IAAmB,GAAG,EAAX,MAAM,CACjB;IAAoB,MAAM,AAA1B,CACA,EADW,MAAM,CACjB;IAAsB,OAAO,AAA7B,CACA,EADW,MAAM,EAAE,CACnB;IAAoB,UAAU,AAA9B,CACA,EADW,MAAM,CACjB;IAAoB,UAAU,AAA9B,CACA,EADW,MAAM,CACjB;IAAoB,OAAO,AAA3B,CACA,EADW,MAAM,CACjB;IAAsB,WAAW,AAAjC,CACA,EADW,MAAM,EAAE,CACnB;IAAsB,SAAS,AAA/B,CACA,EADW,MAAM,EAAE,CACnB;IAAqB,OAAO,AAA5B,CACA,EADW,OAAO,CAClB;IAAoB,KAAK,AAAzB,CACA,EADW,MAAM,CACjB;IAAoB,MAAM,AAA1B,CACA,EADW,MAAM,CACjB;IAAoB,SAAS,AAA7B,CACA,EADW,MAAM,CACjB;IAAoB,OAAO,AAA3B,CACA,EADW,MAAM,CACjB;IAAoB,KAAK,AAAzB,CAA0B,EAAf,MAAM,CAAS;CAAA,CAAA;AA5B7B;;;;GAIG;AAEH;;;;;GAKG;AAEH;;;;;;;;;;;;;;;;GAgBG;AAEH;;;;;;GAMG;AACH,MAAM,CAAC,OAAO,UAAU,YAAY,CAAE,MAAM,AAJzC,CACA,EADQ,MAIiC,EAAE,SAAS,AAHpD,CACA,EADQ,MAG4C,GAF1C,SAAS,CAmGrB"}
\ No newline at end of file