diff --git a/client.go b/client.go
index 8078b4a..99d18fd 100644
--- a/client.go
+++ b/client.go
@@ -24,8 +24,16 @@ type Query interface {
 	Payload() ([]byte, error)
 }
 
-//Client is http.Client for Aozora API Server
-type Client struct {
+//Client interface
+type Client interface {
+	Marketplace() string
+	PartnerTag() string
+	PartnerType() string
+	Request(Query) ([]byte, error)
+}
+
+//client is http.Client for Aozora API Server
+type client struct {
 	server     *Server
 	client     *http.Client
 	ctx        context.Context
@@ -35,21 +43,21 @@ type Client struct {
 }
 
 //Marketplace returns name of Marketplace parameter for PA-API v5
-func (c *Client) Marketplace() string {
+func (c *client) Marketplace() string {
 	return c.server.Marketplace()
 }
 
 //PartnerTag returns PartnerTag parameter for PA-API v5
-func (c *Client) PartnerTag() string {
+func (c *client) PartnerTag() string {
 	return c.partnerTag
 }
 
 //PartnerType returns PartnerType parameter for PA-API v5
-func (c *Client) PartnerType() string {
+func (c *client) PartnerType() string {
 	return defaultPartnerType
 }
 
-func (c *Client) Request(q Query) ([]byte, error) {
+func (c *client) Request(q Query) ([]byte, error) {
 	payload, err := q.Payload()
 	if err != nil {
 		return nil, errs.Wrap(err, "", errs.WithContext("Operation", q.Operation().String()))
@@ -61,7 +69,7 @@ func (c *Client) Request(q Query) ([]byte, error) {
 	return b, nil
 }
 
-func (c *Client) post(cmd Operation, payload []byte) ([]byte, error) {
+func (c *client) post(cmd Operation, payload []byte) ([]byte, error) {
 	dt := NewTimeStamp(time.Now())
 	u := c.server.URL(cmd.Path())
 	hds := newHeaders(c.server, cmd, dt)
@@ -95,7 +103,7 @@ func (c *Client) post(cmd Operation, payload []byte) ([]byte, error) {
 	return body, nil
 }
 
-func (c *Client) authorization(sig string, hds *headers) string {
+func (c *client) authorization(sig string, hds *headers) string {
 	buf := bytes.Buffer{}
 	buf.WriteString(c.server.HMACAlgorithm())
 	buf.WriteString(" Credential=")
@@ -107,7 +115,7 @@ func (c *Client) authorization(sig string, hds *headers) string {
 	return buf.String()
 }
 
-func (c *Client) signiture(signed string, hds *headers) string {
+func (c *client) signiture(signed string, hds *headers) string {
 	dateKey := hmacSHA256([]byte("AWS4"+c.secretKey), []byte(hds.dt.StringDate()))
 	regionKey := hmacSHA256(dateKey, []byte(c.server.Region()))
 	serviceKey := hmacSHA256(regionKey, []byte(c.server.ServiceName()))
@@ -115,7 +123,7 @@ func (c *Client) signiture(signed string, hds *headers) string {
 	return hex.EncodeToString(hmacSHA256(requestKey, []byte(signed)))
 }
 
-func (c *Client) signedString(hds *headers, payload []byte) string {
+func (c *client) signedString(hds *headers, payload []byte) string {
 	return strings.Join(
 		[]string{
 			c.server.HMACAlgorithm(),
@@ -127,7 +135,7 @@ func (c *Client) signedString(hds *headers, payload []byte) string {
 	)
 }
 
-func (c *Client) canonicalRequest(hds *headers, payload []byte) string {
+func (c *client) canonicalRequest(hds *headers, payload []byte) string {
 	request := []string{"POST", hds.cmd.Path(), "", hds.values(), "", hds.list(), hashedString(payload)}
 	return strings.Join(request, "\n")
 }
@@ -181,7 +189,7 @@ func (h *headers) values() string {
 	return strings.Join(list, "\n")
 }
 
-/* Copyright 2019 Spiegel
+/* Copyright 2019,2020 Spiegel
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
diff --git a/client_test.go b/client_test.go
index cc09da3..32fa5e3 100644
--- a/client_test.go
+++ b/client_test.go
@@ -40,17 +40,17 @@ func TestClient(t *testing.T) {
 		},
 	}
 	for _, tc := range testCases {
-		client := New().CreateClient(tc.partnerTag, tc.accessKey, tc.secretKey)
-		if client.Marketplace() != tc.marketplace {
-			t.Errorf("Client.Marketplace() is \"%v\", want \"%v\"", client.Marketplace(), tc.marketplace)
+		c := New().CreateClient(tc.partnerTag, tc.accessKey, tc.secretKey)
+		if c.Marketplace() != tc.marketplace {
+			t.Errorf("Client.Marketplace() is \"%v\", want \"%v\"", c.Marketplace(), tc.marketplace)
 		}
-		if client.PartnerTag() != tc.partnerTag {
-			t.Errorf("Client.PartnerTag() is \"%v\", want \"%v\"", client.PartnerTag(), tc.partnerTag)
+		if c.PartnerTag() != tc.partnerTag {
+			t.Errorf("Client.PartnerTag() is \"%v\", want \"%v\"", c.PartnerTag(), tc.partnerTag)
 		}
-		if client.PartnerType() != tc.partnerType {
-			t.Errorf("Client.PartnerType() is \"%v\", want \"%v\"", client.PartnerType(), tc.partnerType)
+		if c.PartnerType() != tc.partnerType {
+			t.Errorf("Client.PartnerType() is \"%v\", want \"%v\"", c.PartnerType(), tc.partnerType)
 		}
-		hds := newHeaders(client.server, GetItems, tc.date)
+		hds := newHeaders(c.(*client).server, GetItems, tc.date)
 		if hds.get("Content-Encoding") != tc.contentEncoding {
 			t.Errorf("headers.get(\"Content-Encoding\") is \"%v\", want \"%v\"", hds.get("Content-Encoding"), tc.contentEncoding)
 		}
@@ -63,22 +63,22 @@ func TestClient(t *testing.T) {
 		if hds.get("X-Amz-Target") != tc.xAmzTarget {
 			t.Errorf("headers.get(\"X-Amz-Target\") is \"%v\", want \"%v\"", hds.get("X-Amz-Target"), tc.xAmzTarget)
 		}
-		str := client.signedString(hds, tc.payload)
+		str := c.(*client).signedString(hds, tc.payload)
 		if str != tc.sigedText {
 			t.Errorf("Client.signedString() is \"%v\", want \"%v\"", str, tc.sigedText)
 		}
-		sig := client.signiture(str, hds)
+		sig := c.(*client).signiture(str, hds)
 		if sig != tc.sig {
 			t.Errorf("Client.signiture() is \"%v\", want \"%v\"", sig, tc.sig)
 		}
-		auth := client.authorization(sig, hds)
+		auth := c.(*client).authorization(sig, hds)
 		if auth != tc.authorization {
 			t.Errorf("Client.authorization() is \"%v\", want \"%v\"", auth, tc.authorization)
 		}
 	}
 }
 
-/* Copyright 2019 Spiegel
+/* Copyright 2019,2020 Spiegel
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
diff --git a/go.mod b/go.mod
index 9cfe2bb..01bcfd8 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,4 @@ module github.com/spiegel-im-spiegel/pa-api
 
 go 1.13
 
-require github.com/spiegel-im-spiegel/errs v0.3.3
+require github.com/spiegel-im-spiegel/errs v0.3.4
diff --git a/go.sum b/go.sum
index 3311155..2437e58 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,2 @@
-github.com/spiegel-im-spiegel/errs v0.3.3 h1:fFjMj6C45bY9z8ByckmrTwSgUwi/zBL/dgksIEzoHYA=
-github.com/spiegel-im-spiegel/errs v0.3.3/go.mod h1:NwHSe6m3oAhRj2bAkkbzz9xAffIDNcP9uTdyJd9fJNs=
+github.com/spiegel-im-spiegel/errs v0.3.4 h1:lqiLtlT2ex7zpYS+WNWFuFX2x/o8RnG/UeaA8CFRYeE=
+github.com/spiegel-im-spiegel/errs v0.3.4/go.mod h1:NwHSe6m3oAhRj2bAkkbzz9xAffIDNcP9uTdyJd9fJNs=
diff --git a/server.go b/server.go
index 19d5f25..cdb14e1 100644
--- a/server.go
+++ b/server.go
@@ -130,14 +130,14 @@ func (s *Server) ContentEncoding() string {
 }
 
 //ClientOptFunc type is self-referential function type for Server.CreateClient method. (functional options pattern)
-type ClientOptFunc func(*Client)
+type ClientOptFunc func(*client)
 
 //CreateClient method returns an Client instance with associate-tag, access-key, secret-key, and other options.
-func (s *Server) CreateClient(associateTag, accessKey, secretKey string, opts ...ClientOptFunc) *Client {
+func (s *Server) CreateClient(associateTag, accessKey, secretKey string, opts ...ClientOptFunc) Client {
 	if s == nil {
 		s = New()
 	}
-	cli := &Client{
+	cli := &client{
 		server:     s,
 		client:     nil,
 		ctx:        nil,
@@ -160,7 +160,7 @@ func (s *Server) CreateClient(associateTag, accessKey, secretKey string, opts ..
 //WithContext function returns ClientOptFunc function value.
 //This function is used in Server.CreateClient method that represents context.Context.
 func WithContext(ctx context.Context) ClientOptFunc {
-	return func(c *Client) {
+	return func(c *client) {
 		if c != nil {
 			c.ctx = ctx
 		}
@@ -169,20 +169,20 @@ func WithContext(ctx context.Context) ClientOptFunc {
 
 //WithHttpClient function returns ClientOptFunc function value.
 //This function is used in Server.CreateClient method that represents http.Client.
-func WithHttpClient(client *http.Client) ClientOptFunc {
-	return func(c *Client) {
+func WithHttpClient(hc *http.Client) ClientOptFunc {
+	return func(c *client) {
 		if c != nil {
-			c.client = client
+			c.client = hc
 		}
 	}
 }
 
 //DefaultClient function returns an default Client instance with associate-tag, access-key, and secret-key parameters.
-func DefaultClient(associateTag, accessKey, secretKey string) *Client {
+func DefaultClient(associateTag, accessKey, secretKey string) Client {
 	return New().CreateClient(associateTag, accessKey, secretKey)
 }
 
-/* Copyright 2019 Spiegel and contributors
+/* Copyright 2019,2020 Spiegel and contributors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.