X-B3-Flags warn

Hi,
Im using KONG 1.2.1 in production and i´ve start seeing a lot of warn about:
[kong] codec.lua:34 [zipkin] x-b3-flags header invalid; ignoring.,

code: https://github.com/Kong/kong-plugin-zipkin/blob/master/kong/plugins/zipkin/codec.lua#L31
zipkin header: https://github.com/openzipkin/b3-propagation#why-is-debug-encoded-as-x-b3-flags-1

Who can i disable this?