sizeof in #if Compiler statement not working

Hallo,

I use RealView compiler for c
I like to assert a statement by procompiler

Writing:

int i;
const int Ass_FR = sizeof(i);

#if Ass_FR != 5
  #error "Error!!!"
#endif


is working.

but:

int i;

#if sizeof(i) != 5
  #error "Error!!!"
#endif


not.

He says:
error: #59: function call is not allowed in a constant expression

Is this a bug?