Minus Front/Exclude Pathfinder Issue

Hello!

We are trying to cut out the shape at the center of these lines, but the Minus Front/Exclude Pathfinder tools give the response "The filter produced no results. Please select two intersecting paths." The lines were created with the Blend tool. The shape at the middle is just two rectangles united. We've tried expanding the lines and making them a compund path, but it still doesn't work.

How do we get this to work properly?

Thanks!