diff --git a/upnp.go b/upnp.go
index 269ac7ed..86a2748c 100644
--- a/upnp.go
+++ b/upnp.go
@@ -110,8 +110,7 @@ func Discover() (nat NAT, err error) {
 		// HTTP header field names are case-insensitive.
 		// http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2
 		locString := "\r\nlocation: "
-		answer = strings.ToLower(answer)
-		locIndex := strings.Index(answer, locString)
+		locIndex := strings.Index(strings.ToLower(answer), locString)
 		if locIndex < 0 {
 			continue
 		}