Fill command to destroy portals with air = corrupted area

I ran the following command while standing next to an activated obsidian gate:
^{/fill ~-6 ~-6 ~-6 ~6 ~6 ~6 air 0 destroy portal}
It destroyed everything in a 6x6x6 area and placed air. No big deal....

However now...
I can't successfully run the /fill command using oldblockhandle replace in that 6x6x6 area.

Whenever I run the following command in the 6x6x6 area
^{/fill ~-6 ~-6 ~-6 ~6 ~6 ~6 air 0 replace (anyblockhere)}

The command block output always reads:
^{The number you have entered (-5.0) is too small, it must be at least 0}

I know I didn't enter any -5's, so I'm guessing this is a bug.