Grey project monitor when adding pan and zoom effect

I'm using Kdenlive 0.8 on Slackware64 13.37 with MLT 0.7.2.

When I try to add the "Pan and Zoom" video effect the Project Monitor becomes gray as soon as I add it to the video track, if I disable the effect I get the video again on the project monitor. Other video effects work fine but that one is causing issues.

Does anyone know what the problem could be?

Thanks.