NSButton's images lose transparency on selection

I have edited some NSButtons in my Mac OS X application and set a custom image to them. The images are in png format with transparency. When I sele开发者_如何学Goct a button, the background of the image goes white.

Does anyone know of a way to fix this?

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The issue was that I was customising a 'Bevel' button. I changed the button bezel to 'Square' and changed the button type to 'Momentary Change'.

Hope this helps some people in the future!