diff --git a/lib/application.js b/lib/application.js
index 6415444da..46f43134e 100644
--- a/lib/application.js
+++ b/lib/application.js
@@ -10,7 +10,6 @@ const debug = require('debug')('koa:application');
 const onFinished = require('on-finished');
 const response = require('./response');
 const compose = require('koa-compose');
-const isJSON = require('koa-is-json');
 const context = require('./context');
 const request = require('./request');
 const statuses = require('statuses');
@@ -225,9 +224,10 @@ function respond(ctx) {
     return res.end();
   }
 
-  if ('HEAD' == ctx.method) {
-    if (!res.headersSent && isJSON(body)) {
-      ctx.length = Buffer.byteLength(JSON.stringify(body));
+  if ('HEAD' === ctx.method) {
+    if (!res.headersSent && !ctx.response.has('Content-Length')) {
+      const { length } = ctx.response;
+      if (Number.isInteger(length)) ctx.length = length;
     }
     return res.end();
   }
diff --git a/package.json b/package.json
index ef301c216..708e89133 100644
--- a/package.json
+++ b/package.json
@@ -40,7 +40,6 @@
     "is-generator-function": "^1.0.7",
     "koa-compose": "^4.1.0",
     "koa-convert": "^1.2.0",
-    "koa-is-json": "^1.0.0",
     "on-finished": "^2.3.0",
     "only": "~0.0.2",
     "parseurl": "^1.3.2",
diff --git a/test/application/respond.js b/test/application/respond.js
index 14f13211d..d254ead6e 100644
--- a/test/application/respond.js
+++ b/test/application/respond.js
@@ -173,6 +173,26 @@ describe('app.respond', () => {
       assert(!res.text);
     });
 
+    it('should keep stream header if set manually', async() => {
+      const app = new Koa();
+
+      const { length } = fs.readFileSync('package.json');
+
+      app.use(ctx => {
+        ctx.length = length;
+        ctx.body = fs.createReadStream('package.json');
+      });
+
+      const server = app.listen();
+
+      const res = await request(server)
+        .head('/')
+        .expect(200);
+
+      assert.equal(res.header['content-length'], length);
+      assert(!res.text);
+    });
+
     it('should respond with a 404 if no body was set', () => {
       const app = new Koa();