The type or namespace name `IO' does not exist in the namespace `NM.System'. blah blah

Yeah I know why this comes up but I don't udnerstand why it does at the same time. This is the full error:

"The type or namespace name `IO' does not exist in the namespace `NM.System'. Are you missing an assembly reference?"

This comes up on this line but I bet if I fix it it will come up to anywhere System is used:

System.IO.Stream stream

Now as you can see I hard typed namespace and stream, thats everywhere where I use IO, but unity complain about my own namespace NM.System.

Shouldn't this work fine, NM.System isn't System and compiler should see the difference, any idea why this happen? Probably I'm screwed and have to find another namespace.