I need help with mob that plants mushrooms?

I am trying to make a mob that randomly plants mushrooms on ground. One problem is that when it stands on a block with a block that doesn't fully cover the ground (like fence), it may still place a mushroom on that block, removing a fence.
What should I do to prevent mob from placing mushrooms on blocks covered by fence-like blocks?

Here's the code I've got for it:
https://sta.sh/047t3p1lxxm

Please help!