diff --git a/lib/response.js b/lib/response.js
index f965e539dd2..d14e3900e15 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -673,7 +673,11 @@ res.header = function header(field, val) {
       if (Array.isArray(value)) {
         throw new TypeError('Content-Type cannot be set to an Array');
       }
-      value = mime.contentType(value)
+      var contentType = mime.contentType(value);
+      if (contentType === false) {
+        throw new TypeError('invalid content type');
+      }
+      value = contentType;
     }
 
     this.setHeader(field, value);
diff --git a/test/res.set.js b/test/res.set.js
index 04511c1c95f..84631ba9879 100644
--- a/test/res.set.js
+++ b/test/res.set.js
@@ -31,6 +31,19 @@ describe('res', function(){
       .expect('X-Number', '123')
       .expect(200, 'string', done);
     })
+
+    it('should throw when Content-Type is invalid', function (done) {
+      var app = express()
+
+      app.use(function (req, res) {
+        res.set('Content-Type', 'not-a-real-content-type')
+        res.end()
+      })
+
+      request(app)
+      .get('/')
+      .expect(500, /TypeError: invalid content type/, done)
+    })
   })
 
   describe('.set(field, values)', function(){