how to cover contour lines that extends beyond a polygon?

I first plot a color "contourf", and then plot one of the contour lines by using "contour". Because some of the contours extends beyond the polygon (geographic region), I tried to use "patch" or "fill" to cover the contours beyond the boundary (as suggested by friends in another question). Here is the problem:

It only covers the color contourf plots, but does not cover the contour lines plotted by "contour".

What am I missing?