Fire Stream Blocked by Flash .... what does it mean?!?!

[PXL]

This is incredibly unclear ...
I just spent skill points buying the skill:

Flame Flow
Description: Each fire stream deals 1.1M damage per second to the monsters it touches.

But it's darkened out on my run through screen and says "Fire Stream Blocked by Flash"


Absolutely no idea what that means ........

Any help?

[Darenkel]

The second skill on the Wisdom tree reduces the mana cost of Fire Flow at the cost of 1 stream per level.