diff --git a/README.md b/README.md
index 4f23789..ce27082 100644
--- a/README.md
+++ b/README.md
@@ -1,24 +1,23 @@
-# JSON specified HTTP client
+# JSON-specialized HTTP client
 
 [![PkgGoDev](https://pkg.go.dev/badge/github.com/koron-go/jsonhttpc)](https://pkg.go.dev/github.com/koron-go/jsonhttpc)
 [![Actions/Go](https://github.com/koron-go/jsonhttpc/workflows/Go/badge.svg)](https://github.com/koron-go/jsonhttpc/actions?query=workflow%3AGo)
 [![Go Report Card](https://goreportcard.com/badge/github.com/koron-go/jsonhttpc)](https://goreportcard.com/report/github.com/koron-go/jsonhttpc)
 
-Package jsonhttpc provides JSON specialized HTTP Client.
+Package jsonhttpc provides a way to easily send and receive HTTP requests with JSON bodies.
 
-*   request and response is encoded/decoded as JSON automatically.
-*   bit convenient for repeating requests.
+*   Request and response is encoded/decoded as JSON automatically.
+*   Bit handy for repeating requests.
     *   specify base URL - `WithBaseURL()`
     *   specify HTTP Client - `WithClient()`
     *   specify HTTP header - `WithHeader()`
-*   tips to customize requests - `Do()`
+*   Tips to customize requests - for `Do()`
     *   `ContentType() string` on `body` overrides "Content-Type" header.
         (default is "application/json")
-*   error response is decoded as JSON - `Error`
+*   Responses are automatically JSON-decoded even in case of errors - `Error`
     *   Most of JSON properties are put into
         `Properties map[string]interface{}`
-    *   There are some fields for
-        [RFC7808 Problem Details for HTTP APIs][rfc7808]
+    *   Error responses that follow [RFC7808 Problem Details for HTTP APIs][rfc7808] are a bit easier to handle.
 
 (Japanese)
 
@@ -27,13 +26,19 @@ Package jsonhttpc provides JSON specialized HTTP Client.
     *   ベースURLを設定できる `WithBaseURL()`
     *   HTTP Clientを設定できる `WithClient()`
     *   ヘッダーを設定できる `WithHeader()`
-*   リクエストをカスタマイズするtipsがあります `Do()` 
+*   リクエストをカスタマイズするtipsがあります - for `Do()` 
     *   `body` に `ContentType() string` を実装するとContent-Typeヘッダーを変更
         できます (デフォルトは `application/json`)
 *   エラーの際もレスポンスは自動的にJSONデコードされます
     *   `Error.Properties` に `map[string]interface{}` で入ります
     *   [RFC7808 Problem Details for HTTP APIs][rfc7808] は少し楽できます
 
-## How to use
-
 [rfc7808]:https://tools.ietf.org/html/rfc7807
+
+## Install and update
+
+```console
+$ go get github.com/koron-go/jsonhttpc@latest
+```
+
+## How to use
diff --git a/client.go b/client.go
index d827f6e..03ee81c 100644
--- a/client.go
+++ b/client.go
@@ -24,6 +24,8 @@ type Client struct {
 }
 
 // New creates a new Client with base URL.
+// You can pass nil to baseURL, in which case you should pass a complete URL to
+// the pathOrURL argument of Do() function.
 func New(baseURL *url.URL) *Client {
 	return &Client{
 		u: baseURL,
@@ -66,13 +68,6 @@ func (c *Client) WithHeader(h http.Header) *Client {
 // This returns an error when `pathOrURL` starts with "jsonhttpc.Parse error: "
 // to treat `Path()`'s failure as error.
 func (c *Client) Do(ctx context.Context, method, pathOrURL string, body, receiver interface{}) error {
-	if strings.HasPrefix(pathOrURL, "jsonhttpc.Parse error: ") {
-		return errors.New(pathOrURL)
-	}
-	if ctx == nil {
-		ctx = context.Background()
-	}
-
 	// prepare the request.
 	req, err := c.newRawRequest(ctx, method, pathOrURL, body)
 	if err != nil {
@@ -134,6 +129,9 @@ func (c *Client) newRawRequest(ctx context.Context, method, pathOrURL string, bo
 }
 
 func (c *Client) parseURL(pathOrURL string) (*url.URL, error) {
+	if strings.HasPrefix(pathOrURL, "jsonhttpc.Parse error: ") {
+		return nil, errors.New(pathOrURL)
+	}
 	if c.u != nil {
 		return c.u.Parse(pathOrURL)
 	}
diff --git a/error.go b/error.go
index 7ee2dd6..a76f411 100644
--- a/error.go
+++ b/error.go
@@ -111,6 +111,7 @@ func parseError(r *http.Response) (*Error, error) {
 	return er, nil
 }
 
+// Error returns error message.
 func (er *Error) Error() string {
 	return fmt.Sprintf("error: status=%q props=%#v", er.Status, er.Properties)
 }