PGraphics clear() method?

Hi, I have problems clearing a PGraphics layer. Using Java mode, it works perfect, no more with Android mode. I've tried clear(), background(0,0), none works... The only way I found is to recreate the layer... Why is that?

My code, trimmed down to this problem, is like:

        PGraphics layer;
        //only debug version works
        boolean debug=true;
        void setup() {
          size(800, 800);
          layer=createGraphics(width, height);
          layer.beginDraw();
          layer.background(0, 0);
          layer.endDraw();
        }
        void draw() {
          background(128);
          layer.beginDraw();
          layer.ellipse(mouseX, mouseY, 50, 50);
          layer.endDraw();
          image(layer, 0, 0);
        }
        void mousePressed() {
          if (debug) {
            layer=createGraphics(width, height);
            layer.beginDraw();
            layer.background(0, 0);
            layer.endDraw();
          } else {
            layer.beginDraw();
            layer.clear();
            layer.endDraw();
          }
        }

Thanks for your answer.