Ajasent Obsidian Blocks prevent Nether portal appearing

If you build a nether portal frame against an obsidian wall and then try to light it with the flint the block will catch on fire instead of forming a portal. The blocks causing the problem are the lowest two directly behind where the portal will apear. Placing any other kind of block there does not stop the portal appearing. If you remove one block the portal can then be created on that side but not the other.

Occurs in both creative and survival