diff --git a/cert_error_go120.go b/cert_error.go
similarity index 85%
rename from cert_error_go120.go
rename to cert_error.go
index a3cd315..a775f70 100644
--- a/cert_error_go120.go
+++ b/cert_error.go
@@ -1,9 +1,6 @@
 // Copyright (c) HashiCorp, Inc.
 // SPDX-License-Identifier: MPL-2.0
 
-//go:build go1.20
-// +build go1.20
-
 package retryablehttp
 
 import "crypto/tls"
diff --git a/cert_error_go119.go b/cert_error_go119.go
deleted file mode 100644
index b2b27e8..0000000
--- a/cert_error_go119.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// Copyright (c) HashiCorp, Inc.
-// SPDX-License-Identifier: MPL-2.0
-
-//go:build !go1.20
-// +build !go1.20
-
-package retryablehttp
-
-import "crypto/x509"
-
-func isCertError(err error) bool {
-	_, ok := err.(x509.UnknownAuthorityError)
-	return ok
-}
diff --git a/client.go b/client.go
index 91059f7..7878b4b 100644
--- a/client.go
+++ b/client.go
@@ -266,7 +266,7 @@ func getBodyReaderAndContentLength(rawBody interface{}) (ReaderFunc, int64, erro
 	case io.ReadSeeker:
 		raw := body
 		bodyReader = func() (io.Reader, error) {
-			_, err := raw.Seek(0, 0)
+			_, err := raw.Seek(0, io.SeekStart)
 			return io.NopCloser(raw), err
 		}
 		if lr, ok := raw.(LenReader); ok {
@@ -865,7 +865,7 @@ func Get(url string) (*http.Response, error) {
 
 // Get is a convenience helper for doing simple GET requests.
 func (c *Client) Get(url string) (*http.Response, error) {
-	req, err := NewRequest("GET", url, nil)
+	req, err := NewRequest(http.MethodGet, url, nil)
 	if err != nil {
 		return nil, err
 	}
@@ -879,7 +879,7 @@ func Head(url string) (*http.Response, error) {
 
 // Head is a convenience method for doing simple HEAD requests.
 func (c *Client) Head(url string) (*http.Response, error) {
-	req, err := NewRequest("HEAD", url, nil)
+	req, err := NewRequest(http.MethodHead, url, nil)
 	if err != nil {
 		return nil, err
 	}
@@ -895,7 +895,7 @@ func Post(url, bodyType string, body interface{}) (*http.Response, error) {
 // Post is a convenience method for doing simple POST requests.
 // The bodyType parameter sets the "Content-Type" header of the request.
 func (c *Client) Post(url, bodyType string, body interface{}) (*http.Response, error) {
-	req, err := NewRequest("POST", url, body)
+	req, err := NewRequest(http.MethodPost, url, body)
 	if err != nil {
 		return nil, err
 	}
@@ -915,6 +915,15 @@ func (c *Client) PostForm(url string, data url.Values) (*http.Response, error) {
 	return c.Post(url, "application/x-www-form-urlencoded", strings.NewReader(data.Encode()))
 }
 
+// CloseIdleConnections closes any connections on its Transport which were previously
+// connected from previous requests but are now sitting idle in a "keep-alive" state.
+// It does not interrupt any connections currently in use.
+//
+// This implementation delegate the call to inner HTTPClient.
+func (c *Client) CloseIdleConnections() {
+	c.HTTPClient.CloseIdleConnections()
+}
+
 // StandardClient returns a stdlib *http.Client with a custom Transport, which
 // shims in a *retryablehttp.Client for added retries.
 func (c *Client) StandardClient() *http.Client {
diff --git a/roundtripper.go b/roundtripper.go
index 8c407ad..1901406 100644
--- a/roundtripper.go
+++ b/roundtripper.go
@@ -10,6 +10,9 @@ import (
 	"sync"
 )
 
+// ensure RoundTripper implements http.RoundTripper interface.
+var _ http.RoundTripper = (*RoundTripper)(nil)
+
 // RoundTripper implements the http.RoundTripper interface, using a retrying
 // HTTP client to execute requests.
 //