Programmatically trigger a render() ends up into an empty render target

[fabiopolimeni]

OK, so, I am creating an EffectComposer through through the react version of the post-processing library. I then grab the composer out of the react component, which seems to be valid by the time I try to render.

    let subjectRenderTarget = new WebGLRenderTarget(renderTargetSize.width, renderTargetSize.height, {
      depthTexture: new DepthTexture(renderTargetSize.width, renderTargetSize.height, UnsignedIntType),
      depthBuffer: true,
    });

    let colorImageUrl: string | undefined = undefined;
    {
      gl.setRenderTarget(subjectRenderTarget);
      gl.clear();
      if (!!scene.userData.composer) {
        const composer: EffectComposer = scene.userData.composer;
        console.debug("Rendering with composer", composer);
        composer.render(0.016);
        subjectRenderTarget = composer.outputBuffer;
      } else {
        gl.render(scene, camera);
      }

      if (dataObject.isMasked) {
        const { imageUrl } = await readRenderTargetToDataURL(
          gl,
          subjectRenderTarget,
          renderTargetSize.width,
          renderTargetSize.height,
          targetDimension
        );

        colorImageUrl = imageUrl;
      }
    }

The composer seems fine, and correctly populated.

I was thinking that grab the content of the outputBuffer would give me the rendered scene with the post process applied, but I am just getting an empty image.

Can anyone help me figure what's I am doing wrong?