How do I stop EffectComposer from destroying my transparent background?

var width = window.innerWidth || 1;
var height = window.innerHeight || 1;
var parameters = { minFilter: THREE.LinearFilter, magFilter: THREE.LinearFilter, format: THREE.RGBAFormat, stencilBuffer: false };

var renderTarget = new THREE.WebGLRenderTarget( width, height, parameters );

cmp = new THREE.EffectComposer(r, renderTarget);

If you don't specify a rendertarget then it will create one with THREE.RGBFormat which will make you lose alpha.