Compatibility between stencil buffer and SSAO

Now what I have is a mask and a scene with the stencil buffer. The scene renders only behind the mask. But when I apply SSAO, the ambient occlusion shows on all the scene. The scene out of the mask is not rendered, but the SSAO is rendered, which looks weird. I want the SSAO only shows on the masked area. Are there any ways to make this? Thank you so much!