Lava Source Block

I picked up lava in a bucket and placed that lava into another source block of lava.

What I expected to happen was...:
I expected to empty the bucket into the second source block.

What actually happened was...:
I was unable to place the lava into the second source block. I was able to place it elsewhere, just not where other source blocks of lava were.

Steps to Reproduce:
1. Pick up lava in a bucket
2. Attempt to place lava into another source block of lava.