diff --git a/daemon/exec.go b/daemon/exec.go
index 1151a77c7bb50..08b6ac46ffcd3 100644
--- a/daemon/exec.go
+++ b/daemon/exec.go
@@ -157,7 +157,7 @@ func (d *Daemon) ContainerExecStart(name string, stdin io.ReadCloser, stdout io.
 	logrus.Debugf("starting exec command %s in container %s", ec.ID, c.ID)
 	d.LogContainerEvent(c, "exec_start: "+ec.ProcessConfig.Entrypoint+" "+strings.Join(ec.ProcessConfig.Arguments, " "))
 
-	if ec.OpenStdin {
+	if ec.OpenStdin && stdin != nil {
 		r, w := io.Pipe()
 		go func() {
 			defer w.Close()
diff --git a/integration-cli/docker_api_exec_test.go b/integration-cli/docker_api_exec_test.go
index 51533223c72ca..0c41687c9cf78 100644
--- a/integration-cli/docker_api_exec_test.go
+++ b/integration-cli/docker_api_exec_test.go
@@ -122,6 +122,36 @@ func (s *DockerSuite) TestExecApiStartMultipleTimesError(c *check.C) {
 	startExec(c, execID, http.StatusConflict)
 }
 
+// #20638
+func (s *DockerSuite) TestExecApiStartWithDetach(c *check.C) {
+	name := "foo"
+	dockerCmd(c, "run", "-d", "-t", "--name", name, "busybox", "top")
+	data := map[string]interface{}{
+		"cmd":         []string{"true"},
+		"AttachStdin": true,
+	}
+	_, b, err := sockRequest("POST", fmt.Sprintf("/containers/%s/exec", name), data)
+	c.Assert(err, checker.IsNil, check.Commentf(string(b)))
+
+	createResp := struct {
+		ID string `json:"Id"`
+	}{}
+	c.Assert(json.Unmarshal(b, &createResp), checker.IsNil, check.Commentf(string(b)))
+
+	_, body, err := sockRequestRaw("POST", fmt.Sprintf("/exec/%s/start", createResp.ID), strings.NewReader(`{"Detach": true}`), "application/json")
+	c.Assert(err, checker.IsNil)
+
+	b, err = readBody(body)
+	comment := check.Commentf("response body: %s", b)
+	c.Assert(err, checker.IsNil, comment)
+
+	resp, _, err := sockRequestRaw("GET", "/_ping", nil, "")
+	c.Assert(err, checker.IsNil)
+	if resp.StatusCode != http.StatusOK {
+		c.Fatal("daemon is down, it should alive")
+	}
+}
+
 func createExec(c *check.C, name string) string {
 	_, b, err := sockRequest("POST", fmt.Sprintf("/containers/%s/exec", name), map[string]interface{}{"Cmd": []string{"true"}})
 	c.Assert(err, checker.IsNil, check.Commentf(string(b)))