diff --git a/acme/internal/acme/acme.go b/acme/internal/acme/acme.go
index effc0ec..eb60ba5 100644
--- a/acme/internal/acme/acme.go
+++ b/acme/internal/acme/acme.go
@@ -48,14 +48,14 @@ const LetsEncryptURL = "https://acme-v01.api.letsencrypt.org/directory"
 // 	client := &Client{Key: key}
 //
 type Client struct {
-	// HTTPClient optionally specifies an HTTP client to use
-	// instead of http.DefaultClient.
-	HTTPClient *http.Client
-
 	// Key is the account key used to register with a CA and sign requests.
 	// Key.Public() must return a *rsa.PublicKey or *ecdsa.PublicKey.
 	Key crypto.Signer
 
+	// HTTPClient optionally specifies an HTTP client to use
+	// instead of http.DefaultClient.
+	HTTPClient *http.Client
+
 	// DirectoryURL points to the CA directory endpoint.
 	// If empty, LetsEncryptURL is used.
 	// Mutating this value after a successful call of Client's Discover method
@@ -195,18 +195,18 @@ func (c *Client) FetchCert(ctx context.Context, url string, bundle bool) ([][]by
 	}
 }
 
-// AcceptTOS always returns true to indicate the acceptance of a CA Terms of Service
+// AcceptTOS always returns true to indicate the acceptance of a CA's Terms of Service
 // during account registration. See Register method of Client for more details.
-func AcceptTOS(string) bool { return true }
+func AcceptTOS(tosURL string) bool { return true }
 
 // Register creates a new account registration by following the "new-reg" flow.
 // It returns registered account. The a argument is not modified.
 //
-// The registration may require the caller to agree to the CA Terms of Service (TOS).
+// The registration may require the caller to agree to the CA's Terms of Service (TOS).
 // If so, and the account has not indicated the acceptance of the terms (see Account for details),
 // Register calls prompt with a TOS URL provided by the CA. Prompt should report
 // whether the caller agrees to the terms. To always accept the terms, the caller can use AcceptTOS.
-func (c *Client) Register(a *Account, prompt func(tos string) bool) (*Account, error) {
+func (c *Client) Register(a *Account, prompt func(tosURL string) bool) (*Account, error) {
 	if _, err := c.Discover(); err != nil {
 		return nil, err
 	}
@@ -279,10 +279,10 @@ func (c *Client) Authorize(domain string) (*Authorization, error) {
 
 	var v wireAuthz
 	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
-		return nil, fmt.Errorf("Decode: %v", err)
+		return nil, fmt.Errorf("acme: invalid response: %v", err)
 	}
 	if v.Status != StatusPending {
-		return nil, fmt.Errorf("Unexpected status: %s", v.Status)
+		return nil, fmt.Errorf("acme: unexpected status: %s", v.Status)
 	}
 	return v.authorization(res.Header.Get("Location")), nil
 }
@@ -301,7 +301,7 @@ func (c *Client) GetAuthz(url string) (*Authorization, error) {
 	}
 	var v wireAuthz
 	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
-		return nil, fmt.Errorf("Decode: %v", err)
+		return nil, fmt.Errorf("acme: invalid response: %v", err)
 	}
 	return v.authorization(url), nil
 }
@@ -320,7 +320,7 @@ func (c *Client) GetChallenge(url string) (*Challenge, error) {
 	}
 	v := wireChallenge{URI: url}
 	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
-		return nil, fmt.Errorf("Decode: %v", err)
+		return nil, fmt.Errorf("acme: invalid response: %v", err)
 	}
 	return v.challenge(), nil
 }
@@ -357,7 +357,7 @@ func (c *Client) Accept(chal *Challenge) (*Challenge, error) {
 
 	var v wireChallenge
 	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
-		return nil, fmt.Errorf("Decode: %v", err)
+		return nil, fmt.Errorf("acme: invalid response: %v", err)
 	}
 	return v.challenge(), nil
 }
@@ -500,7 +500,7 @@ func (c *Client) doReg(url string, typ string, acct *Account) (*Account, error)
 		Certificates   string
 	}
 	if err := json.NewDecoder(res.Body).Decode(&v); err != nil {
-		return nil, fmt.Errorf("Decode: %v", err)
+		return nil, fmt.Errorf("acme: invalid response: %v", err)
 	}
 	return &Account{
 		URI:            res.Header.Get("Location"),
@@ -516,7 +516,7 @@ func (c *Client) doReg(url string, typ string, acct *Account) (*Account, error)
 func responseCert(client *http.Client, res *http.Response, bundle bool) ([][]byte, error) {
 	b, err := ioutil.ReadAll(res.Body)
 	if err != nil {
-		return nil, fmt.Errorf("ReadAll: %v", err)
+		return nil, fmt.Errorf("acme: response stream: %v", err)
 	}
 	cert := [][]byte{b}
 	if !bundle {
@@ -526,7 +526,7 @@ func responseCert(client *http.Client, res *http.Response, bundle bool) ([][]byt
 	// append ca cert
 	up := linkHeader(res.Header, "up")
 	if up == "" {
-		return nil, errors.New("rel=up link not found")
+		return nil, errors.New("acme: rel=up link not found")
 	}
 	res, err = client.Get(up)
 	if err != nil {
@@ -580,7 +580,7 @@ func fetchNonce(client *http.Client, url string) (string, error) {
 	defer resp.Body.Close()
 	enc := resp.Header.Get("replay-nonce")
 	if enc == "" {
-		return "", errors.New("nonce not found")
+		return "", errors.New("acme: nonce not found")
 	}
 	return enc, nil
 }
diff --git a/acme/internal/acme/acme_test.go b/acme/internal/acme/acme_test.go
index 3237b09..bb7dd1f 100644
--- a/acme/internal/acme/acme_test.go
+++ b/acme/internal/acme/acme_test.go
@@ -801,6 +801,7 @@ func TestTLSSNI01ChallengeCert(t *testing.T) {
 		t.Errorf("cert.DNSNames[0] != name: %q vs %q", cert.DNSNames[0], name)
 	}
 }
+
 func TestTLSSNI02ChallengeCert(t *testing.T) {
 	const (
 		token = "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"