diff --git a/index.js b/index.js
index 2e757bd..1d10dc1 100644
--- a/index.js
+++ b/index.js
@@ -28,6 +28,9 @@ function formatCacheControl(cc) {
         const v = cc[k];
         parts.push(v === true ? k : k + '=' + v);
     }
+    if (!parts.length) {
+        return undefined;
+    }
     return parts.join(', ');
 }
 
@@ -68,6 +71,7 @@ module.exports = class CachePolicy {
             delete this._rescc['no-store'];
             delete this._rescc['must-revalidate'];
             this._resHeaders = Object.assign({}, this._resHeaders, {'cache-control': formatCacheControl(this._rescc)});
+            delete this._resHeaders['expires'];
             delete this._resHeaders['pragma'];
         }
 
diff --git a/package.json b/package.json
index ac1df20..7fccafc 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "http-cache-semantics",
-  "version": "3.3.2",
+  "version": "3.3.3",
   "description": "Parses Cache-Control headers and friends",
   "main": "index.js",
   "repository": "https://github.com/pornel/http-cache-semantics.git",
diff --git a/test/responsetest.js b/test/responsetest.js
index d69af3f..8a9cf52 100644
--- a/test/responsetest.js
+++ b/test/responsetest.js
@@ -65,6 +65,21 @@ describe('Response headers', function() {
         assert(!cache.responseHeaders()['pragma']);
     });
 
+    it('pre-check poison undefined header', function() {
+        const origCC = 'pre-check=0, post-check=0, no-cache, no-store';
+        const res = {headers:{'cache-control': origCC, expires: 'yesterday!'}};
+        const cache = new CachePolicy(req, res, {ignoreCargoCult:true});
+        assert(cache.stale());
+        assert(cache.storable());
+        assert.equal(cache.maxAge(), 0);
+
+        const cc = cache.responseHeaders()['cache-control'];
+        assert(!cc);
+
+        assert(res.headers['expires']);
+        assert(!cache.responseHeaders()['expires']);
+    });
+
     it('cache with expires', function() {
         const cache = new CachePolicy(req, {headers:{
             'date': new Date().toGMTString(),