diff --git a/discogs.go b/discogs.go
index 5a3f524..d05c3cb 100644
--- a/discogs.go
+++ b/discogs.go
@@ -2,7 +2,6 @@ package discogs
 
 import (
 	"encoding/json"
-	"fmt"
 	"io/ioutil"
 	"net/http"
 	"net/url"
@@ -33,7 +32,7 @@ func NewClient(o *Options) (*Client, error) {
 	header = &http.Header{}
 
 	if o == nil || o.UserAgent == "" {
-		return nil, fmt.Errorf("failed to set user-agent")
+		return nil, ErrUserAgentInvalid
 	}
 
 	header.Add("User-Agent", o.UserAgent)
@@ -68,7 +67,7 @@ func currency(c string) (string, error) {
 	case "":
 		return "USD", nil
 	default:
-		return "", fmt.Errorf("invalid currency abbreviation")
+		return "", ErrCurrencyNotSupported
 	}
 }
 
diff --git a/discogs_test.go b/discogs_test.go
index 00a1624..1f280ec 100644
--- a/discogs_test.go
+++ b/discogs_test.go
@@ -29,3 +29,63 @@ func initDiscogsClient(t *testing.T, options *Options) *Client {
 
 	return client
 }
+
+func TestNewClient(t *testing.T) {
+	tests := map[string]struct {
+		options *Options
+		err     error
+	}{
+		"normal": {&Options{
+			UserAgent: testUserAgent,
+			Currency:  "USD",
+		}, nil},
+		"incorrect user-agent": {&Options{
+			UserAgent: "",
+			Currency:  "USD",
+		}, ErrUserAgentInvalid},
+		"incorrect currency": {&Options{
+			UserAgent: testUserAgent,
+			Currency:  "RUR",
+		}, ErrCurrencyNotSupported},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			if _, err := NewClient(tt.options); err != tt.err {
+				t.Errorf("err got=%s; want=%s", err, tt.err)
+			}
+		})
+	}
+}
+
+func TestCurrency(t *testing.T) {
+	tests := []struct {
+		currency string
+		want     string
+		err      error
+	}{
+		{currency: "", want: "USD", err: nil},
+		{currency: "USD", want: "USD", err: nil},
+		{currency: "GBP", want: "GBP", err: nil},
+		{currency: "EUR", want: "EUR", err: nil},
+		{currency: "CAD", want: "CAD", err: nil},
+		{currency: "AUD", want: "AUD", err: nil},
+		{currency: "JPY", want: "JPY", err: nil},
+		{currency: "CHF", want: "CHF", err: nil},
+		{currency: "MXN", want: "MXN", err: nil},
+		{currency: "BRL", want: "BRL", err: nil},
+		{currency: "NZD", want: "NZD", err: nil},
+		{currency: "SEK", want: "SEK", err: nil},
+		{currency: "ZAR", want: "ZAR", err: nil},
+		{currency: "RUR", want: "", err: ErrCurrencyNotSupported},
+	}
+	for i, tt := range tests {
+		cur, err := currency(tt.currency)
+		if err != tt.err {
+			t.Errorf("#%d err got=%s; want=%s", i, err, tt.err)
+		}
+		if cur != tt.want {
+			t.Errorf("#%d currency got=%s; want=%s", i, cur, tt.want)
+		}
+	}
+}
diff --git a/errors.go b/errors.go
index 8ddbab4..00a2fea 100644
--- a/errors.go
+++ b/errors.go
@@ -2,39 +2,20 @@ package discogs
 
 import (
 	"fmt"
+	"strings"
 )
 
-// APIError represents a Discogs API Error response
-type APIError struct {
-	Message string `json:"message"`
+// Error represents a Discogs API error
+type Error struct {
+	Message string
 }
 
-// Error ...
-func (e APIError) Error() string {
-	if e.Message != "" {
-		return fmt.Sprintf("discogs: %v", e.Message)
-	}
-	return ""
+func (e *Error) Error() string {
+	return fmt.Sprintf("%s", strings.ToLower(e.Message))
 }
 
-// Empty returns true if empty. Otherwise, at least 1 error message/code is
-// present and false is returned.
-func (e APIError) Empty() bool {
-	if e.Message == "" {
-		return true
-	}
-	return false
-}
-
-// relevantError returns any non-nil http-related error (creating the request,
-// getting the response, decoding) if any. If the decoded apiError is non-zero
-// the apiError is returned. Otherwise, no errors occurred, returns nil.
-func relevantError(httpError error, apiError APIError) error {
-	if httpError != nil {
-		return httpError
-	}
-	if apiError.Empty() {
-		return nil
-	}
-	return apiError
-}
+// APIErrors
+var (
+	ErrCurrencyNotSupported = &Error{"currency does not supported"}
+	ErrUserAgentInvalid     = &Error{"invalid user-agent"}
+)
diff --git a/examples/discogs_example.go b/examples/discogs_example.go
index 59aac15..1217163 100644
--- a/examples/discogs_example.go
+++ b/examples/discogs_example.go
@@ -9,7 +9,7 @@ import (
 func main() {
 	d, err := discogs.NewClient(&discogs.Options{
 		UserAgent: "TestDiscogsClient/0.0.1 +http://example.com",
-		Currency:  "EUR",
+		Currency:  "AAA",
 		Token:     "",
 	})
 	if err != nil {