Why does "x" as a local variable not shadow an inst-var "x"?

Object subclass: #Foo
 instanceVariableNames: 'x' <-- sometimes compile error here

Foo>>m
 | x | <-- sometimes compile error here

I tried to add an inst-var to one of my classes, and was forced to update 
local vars in some methods (or to use a different inst-var name).

Thanks