diff --git a/plugin.js b/plugin.js
index 01a64ca..0b0a5d5 100644
--- a/plugin.js
+++ b/plugin.js
@@ -70,31 +70,30 @@ function onReqHandlerWrapper (fastify, hook) {
 }
 
 function setCookies (reply) {
-  let setCookie = reply.getHeader('Set-Cookie')
-  const setCookieIsUndefined = setCookie === undefined
+  const setCookieHeader = reply.getHeader('Set-Cookie')
+  let setCookie
 
-  /* istanbul ignore else */
-  if (setCookieIsUndefined) {
+  if (setCookieHeader === undefined) {
     if (reply[kReplySetCookies].size === 1) {
-      for (const c of reply[kReplySetCookies].values()) {
-        reply.header('Set-Cookie', cookie.serialize(c.name, c.value, c.opts))
-      }
-
+      // Fast path for single cookie
+      const c = reply[kReplySetCookies].values().next().value
+      reply.header('Set-Cookie', cookie.serialize(c.name, c.value, c.opts))
       reply[kReplySetCookies].clear()
-
       return
     }
 
     setCookie = []
-  } else if (typeof setCookie === 'string') {
-    setCookie = [setCookie]
+  } else if (typeof setCookieHeader === 'string') {
+    setCookie = [setCookieHeader]
+  } else {
+    setCookie = setCookieHeader
   }
 
   for (const c of reply[kReplySetCookies].values()) {
     setCookie.push(cookie.serialize(c.name, c.value, c.opts))
   }
 
-  if (!setCookieIsUndefined) reply.removeHeader('Set-Cookie')
+  if (setCookieHeader !== undefined) reply.removeHeader('Set-Cookie')
   reply.header('Set-Cookie', setCookie)
   reply[kReplySetCookies].clear()
 }
diff --git a/test/cookie.test.js b/test/cookie.test.js
index b05faad..e1f0b47 100644
--- a/test/cookie.test.js
+++ b/test/cookie.test.js
@@ -132,6 +132,38 @@ test('should set multiple cookies', (t) => {
   })
 })
 
+test('should set multiple cookies (an array already exists)', (t) => {
+  t.plan(10)
+  const fastify = Fastify()
+  fastify.register(plugin)
+
+  fastify.get('/test1', (req, reply) => {
+    reply
+      .header('Set-Cookie', ['bar=bar'])
+      .setCookie('foo', 'foo', { path: '/' })
+      .send({ hello: 'world' })
+  })
+
+  fastify.inject({
+    method: 'GET',
+    url: '/test1'
+  }, (err, res) => {
+    t.error(err)
+    t.equal(res.statusCode, 200)
+    t.same(JSON.parse(res.body), { hello: 'world' })
+
+    const cookies = res.cookies
+    t.equal(cookies.length, 2)
+    t.equal(cookies[0].name, 'bar')
+    t.equal(cookies[0].value, 'bar')
+    t.equal(cookies[0].path, undefined)
+
+    t.equal(cookies[1].name, 'foo')
+    t.equal(cookies[1].value, 'foo')
+    t.equal(cookies[1].path, '/')
+  })
+})
+
 test('cookies get set correctly with millisecond dates', (t) => {
   t.plan(8)
   const fastify = Fastify()
diff --git a/types/plugin.d.ts b/types/plugin.d.ts
index 8cc7a9c..0f973fd 100644
--- a/types/plugin.d.ts
+++ b/types/plugin.d.ts
@@ -65,11 +65,7 @@ declare module "fastify" {
      * @param value Cookie value
      * @param options Serialize options
      */
-    setCookie(
-      name: string,
-      value: string,
-      options?: fastifyCookie.CookieSerializeOptions
-    ): this;
+    setCookie: setCookieWrapper;
 
     /**
      * @alias setCookie