diff --git a/websocket/server.go b/websocket/server.go
index 8135716..315c7fb 100644
--- a/websocket/server.go
+++ b/websocket/server.go
@@ -114,7 +114,7 @@ func (u *Upgrader) Upgrade(ctx *fasthttp.RequestCtx, responseHeader *fasthttp.Re
 	}
 
 	if nil != responseHeader {
-		if v := responseHeader.Peek("Sec-Websocket-Extensions"); nil == v {
+		if v := responseHeader.Peek("Sec-Websocket-Extensions"); nil != v {
 			cb(u.returnError(ctx, fasthttp.StatusInternalServerError, "websocket: application specific 'Sec-Websocket-Extensions' headers are unsupported"))
 			return
 		}