Can not make vertical cobblestone generator because of bug

What I expected to happen was...:
put water on top of lava and the two mix to create either cobblestone or obsidian

What actually happened was...:
water floated on top of lava in mid air

Steps to Reproduce:
1. make lava flow into a specified area of your choice
2. put water flowing down on top of lava in the specified area
3. make sure both water and lava are flowing
4. error