diff --git a/Makefile b/Makefile
index 2a1e304..9c73383 100644
--- a/Makefile
+++ b/Makefile
@@ -1,15 +1,22 @@
-
-.PHONEY: all setup test cover
-
+.PHONEY: all
 all: setup cover
 
+.PHONEY: setup
 setup:
-		go get golang.org/x/tools/cmd/cover
-		go get google.golang.org/appengine/urlfetch
-		go get ./...
+	go get golang.org/x/tools/cmd/cover
+	go get google.golang.org/appengine/urlfetch
+	go get ./...
 
+.PHONEY: test
 test:
-		go test -v ./...
+	go test -v ./...
 
+.PHONEY: cover
 cover:
-		go test -coverprofile=coverage.txt ./...
+	go test -coverprofile=coverage.txt ./...
+
+.PHONEY: generate
+generate:
+	rm -rf ./appstore/mocks/*
+	rm -rf ./playstore/mocks/*
+	go generate ./...
diff --git a/appstore/mocks/mock_appstore.go b/appstore/mocks/appstore.go
similarity index 100%
rename from appstore/mocks/mock_appstore.go
rename to appstore/mocks/appstore.go
diff --git a/appstore/validator.go b/appstore/validator.go
index 301ce59..f68f2b6 100644
--- a/appstore/validator.go
+++ b/appstore/validator.go
@@ -10,7 +10,7 @@ import (
 	"time"
 )
 
-//go:generate mockgen  -destination=mocks/mock_appstore.go -package=mocks github.com/awa/go-iap/appstore IAPClient
+//go:generate mockgen  -destination=mocks/appstore.go -package=mocks github.com/awa/go-iap/appstore IAPClient
 
 const (
 	// SandboxURL is the endpoint for sandbox environment.
diff --git a/playstore/mocks/mock_playstore.go b/playstore/mocks/playstore.go
similarity index 100%
rename from playstore/mocks/mock_playstore.go
rename to playstore/mocks/playstore.go
diff --git a/playstore/validator.go b/playstore/validator.go
index 5a6e1ba..f572029 100644
--- a/playstore/validator.go
+++ b/playstore/validator.go
@@ -8,16 +8,17 @@ import (
 	"crypto/x509"
 	"encoding/base64"
 	"fmt"
-	"google.golang.org/api/option"
 	"net/http"
 	"time"
 
+	"google.golang.org/api/option"
+
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	androidpublisher "google.golang.org/api/androidpublisher/v3"
 )
 
-//go:generate mockgen  -destination=mocks/mock_playstore.go -package=mocks github.com/awa/go-iap/playstore IABProduct,IABSubscription
+//go:generate mockgen  -destination=mocks/playstore.go -package=mocks github.com/awa/go-iap/playstore IABProduct,IABSubscription
 
 // The IABProduct type is an interface for product service
 type IABProduct interface {
@@ -46,7 +47,6 @@ func New(jsonKey []byte) (*Client, error) {
 	c := &http.Client{Timeout: 10 * time.Second}
 	ctx := context.WithValue(context.Background(), oauth2.HTTPClient, c)
 
-
 	conf, err := google.JWTConfigFromJSON(jsonKey, androidpublisher.AndroidpublisherScope)
 	if err != nil {
 		return nil, err
diff --git a/playstore/validator_test.go b/playstore/validator_test.go
index b40ba52..4f90541 100644
--- a/playstore/validator_test.go
+++ b/playstore/validator_test.go
@@ -35,7 +35,7 @@ func TestNew(t *testing.T) {
 	t.Parallel()
 
 	// Exception scenario
-	expected := "oauth2: cannot fetch token: 400 Bad Request\nResponse: {\n  \"error\": \"invalid_grant\",\n  \"error_description\": \"Invalid issuer: Not a service account.\"\n}"
+	expected := "oauth2: cannot fetch token: 400 Bad Request\nResponse: {\"error\":\"invalid_grant\",\"error_description\":\"Invalid JWT: iss field missing.\"}"
 
 	_, err := New(dummyKey)
 	if err == nil || err.Error() != expected {