Created silhouette only shows on sky background

Anyone has a clue to fix this?

Here is the code:

pickedFeatureStyle: () => {
      const handler = new ScreenSpaceEventHandler(scene.canvas);

      if (PostProcessStageLibrary.isSilhouetteSupported(scene)) {
        const silhouetteBlue = PostProcessStageLibrary.createEdgeDetectionStage();
        silhouetteBlue.uniforms.color = Color.ROYALBLUE;
        silhouetteBlue.uniforms.length = 2;
        silhouetteBlue.selected = [];

        scene.postProcessStages.add(
          PostProcessStageLibrary.createSilhouetteStage([
            silhouetteBlue
          ])
        );
        handler.setInputAction((movement) => {
          silhouetteBlue.selected = [];
          const pickedFeature = scene.pick(movement.position);
          if (silhouetteBlue.selected[0] === pickedFeature) {
            return;
          }
          silhouetteBlue.selected = [pickedFeature];
        }, ScreenSpaceEventType.LEFT_CLICK);
      } else {
        console.warn('Silhouette for object is not supported');
      }
    },

And here is the result: