diff --git a/rest/httpx/responses.go b/rest/httpx/responses.go
index 9ec67e5b..28b4ba1b 100644
--- a/rest/httpx/responses.go
+++ b/rest/httpx/responses.go
@@ -61,12 +61,16 @@ func SetErrorHandler(handler func(error) (int, interface{})) {
 
 // WriteJson writes v as json string into w with code.
 func WriteJson(w http.ResponseWriter, code int, v interface{}) {
+	bs, err := json.Marshal(v)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
 	w.Header().Set(ContentType, ApplicationJson)
 	w.WriteHeader(code)
 
-	if bs, err := json.Marshal(v); err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
-	} else if n, err := w.Write(bs); err != nil {
+	if n, err := w.Write(bs); err != nil {
 		// http.ErrHandlerTimeout has been handled by http.TimeoutHandler,
 		// so it's ignored here.
 		if err != http.ErrHandlerTimeout {
diff --git a/rest/httpx/responses_test.go b/rest/httpx/responses_test.go
index c886ae2e..9a553cd0 100644
--- a/rest/httpx/responses_test.go
+++ b/rest/httpx/responses_test.go
@@ -146,6 +146,16 @@ func TestWriteJsonLessWritten(t *testing.T) {
 	assert.Equal(t, http.StatusOK, w.code)
 }
 
+func TestWriteJsonMarshalFailed(t *testing.T) {
+	w := tracedResponseWriter{
+		headers: make(map[string][]string),
+	}
+	WriteJson(&w, http.StatusOK, map[string]interface{}{
+		"Data": complex(0, 0),
+	})
+	assert.Equal(t, http.StatusInternalServerError, w.code)
+}
+
 type tracedResponseWriter struct {
 	headers     map[string][]string
 	builder     strings.Builder
@@ -153,6 +163,7 @@ type tracedResponseWriter struct {
 	code        int
 	lessWritten bool
 	timeout     bool
+	wroteHeader bool
 }
 
 func (w *tracedResponseWriter) Header() http.Header {
@@ -174,5 +185,9 @@ func (w *tracedResponseWriter) Write(bytes []byte) (n int, err error) {
 }
 
 func (w *tracedResponseWriter) WriteHeader(code int) {
+	if w.wroteHeader {
+		return
+	}
+	w.wroteHeader = true
 	w.code = code
 }