diff --git a/oauthproxy.go b/oauthproxy.go
index 1679b11..8cbb4e5 100644
--- a/oauthproxy.go
+++ b/oauthproxy.go
@@ -291,7 +291,7 @@ func (p *OAuthProxy) MakeSessionCookie(req *http.Request, value string, expirati
 		value = cookie.SignedValue(p.CookieSeed, p.CookieName, value, now)
 	}
 	c := p.makeCookie(req, p.CookieName, value, expiration, now)
-	if len(c.Value) > 4096 {
+	if len(c.Value) > 4096-len(p.CookieName) {
 		return splitCookie(c)
 	}
 	return []*http.Cookie{c}