diff --git a/http2/transport.go b/http2/transport.go
index 4f08ccb..da53e83 100644
--- a/http2/transport.go
+++ b/http2/transport.go
@@ -1880,6 +1880,9 @@
 	if err != nil {
 		return nil, err
 	}
+	if !httpguts.ValidHostHeader(host) {
+		return nil, errors.New("http2: invalid Host header")
+	}
 
 	var path string
 	if req.Method != "CONNECT" {