Crop with scripts failed,Help please.

Hi~,

when i use script command 'app.activeDocument.crop(layer.bounds)', error message popout:The command cannot be completed because the affected area is empty or does not overlap the canvas.

before this action, i have set all layers invisible, then:

   layer.visible = true;

   app.activeDocument.activeLayer = layer;

   app.activeDocument.crop(layer.bounds);//this cause error

   _saveImage(ffname);//export layer as png

   layer.visible = false;

What should i do?

Thanks so much