diff --git a/api/client/builder/client.go b/api/client/builder/client.go
index 25ecccb4ecf0..fb2113dd3e77 100644
--- a/api/client/builder/client.go
+++ b/api/client/builder/client.go
@@ -27,7 +27,7 @@ const (
 	postRegisterValidatorPath  = "/eth/v1/builder/validators"
 )
 
-var ErrMalformedHostname = errors.New("hostname must include port, separated by one colon, like example.com:3500")
+var errMalformedHostname = errors.New("hostname must include port, separated by one colon, like example.com:3500")
 
 // ClientOpt is a functional option for the Client type (http.Client wrapper)
 type ClientOpt func(*Client)
@@ -72,7 +72,7 @@ func urlForHost(h string) (*url.URL, error) {
 	// try to parse as host:port
 	host, port, err := net.SplitHostPort(h)
 	if err != nil {
-		return nil, ErrMalformedHostname
+		return nil, errMalformedHostname
 	}
 	return &url.URL{Host: fmt.Sprintf("%s:%s", host, port), Scheme: "http"}, nil
 }