diff --git a/client.go b/client.go
index 2efd835..3be04aa 100644
--- a/client.go
+++ b/client.go
@@ -9,6 +9,7 @@ import (
 	"context"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"net"
@@ -185,6 +186,12 @@ func (d *Dialer) DialContext(ctx context.Context, urlStr string, requestHeader h
 		return nil, nil, errMalformedURL
 	}
 
+	for _, proto := range d.TLSClientConfig.NextProtos {
+		if proto != "http/1.1" {
+			return nil, nil, fmt.Errorf("protocol %q is currently not supported", proto)
+		}
+	}
+
 	req := &http.Request{
 		Method:     http.MethodGet,
 		URL:        u,