diff --git a/jwt.go b/jwt.go
index 0575a44..cd5f2ec 100644
--- a/jwt.go
+++ b/jwt.go
@@ -6,6 +6,12 @@ import (
 	"net/http"
 )
 
+var (
+	ErrMissingConfig   = errors.New("missing configuration")
+	ErrMissingSecret   = errors.New("please provide a shared secret")
+	ErrMissingAuthFunc = errors.New("please provide an auth function")
+)
+
 type Config struct {
 	Secret string
 	Auth   AuthFunc
@@ -14,14 +20,24 @@ type Config struct {
 type AuthFunc func(string, string) (bool, error)
 
 type JWTMiddleware struct {
-	config Config
+	secret string
+	auth   AuthFunc
 }
 
 func NewMiddleware(c *Config) (*JWTMiddleware, error) {
 	if c == nil {
-		return nil, errors.New("missing configuration")
+		return nil, ErrMissingConfig
+	}
+	if c.Secret == "" {
+		return nil, ErrMissingSecret
+	}
+	if c.Auth == nil {
+		return nil, ErrMissingAuthFunc
+	}
+	m := &JWTMiddleware{
+		secret: c.Secret,
+		auth:   c.Auth,
 	}
-	m := &JWTMiddleware{config: *c}
 	return m, nil
 }
 
@@ -37,7 +53,7 @@ func (m *JWTMiddleware) GenerateToken(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		panic(err)
 	}
-	result, err := m.config.Auth(b["email"], b["password"])
+	result, err := m.auth(b["email"], b["password"])
 	if err != nil {
 		panic(err)
 	}
diff --git a/jwt_test.go b/jwt_test.go
index 29facdd..be1aafc 100644
--- a/jwt_test.go
+++ b/jwt_test.go
@@ -31,16 +31,24 @@ func newJWTMiddlewareOrFatal(t *testing.T) *JWTMiddleware {
 
 func TestNewJWTMiddleware(t *testing.T) {
 	middleware := newJWTMiddlewareOrFatal(t)
-	if middleware.config.Secret != "password" {
-		t.Errorf("expected 'password', got %v", middleware.config.Secret)
+	if middleware.secret != "password" {
+		t.Errorf("expected 'password', got %v", middleware.secret)
 	}
 	// TODO: test auth func init
 }
 
 func TestNewJWTMiddlewareNoConfig(t *testing.T) {
-	_, err := NewMiddleware(nil)
-	if err == nil {
-		t.Error("expected configuration error, received none")
+	cases := map[*Config]error{
+		nil:                       ErrMissingConfig,
+		&Config{}:                 ErrMissingSecret,
+		&Config{Auth: authFunc}:   ErrMissingSecret,
+		&Config{Secret: "secret"}: ErrMissingAuthFunc,
+	}
+	for config, jwtErr := range cases {
+		_, err := NewMiddleware(config)
+		if err != jwtErr {
+			t.Errorf("wanted error: %v, got error: %v using config: %v", jwtErr, err, config)
+		}
 	}
 }