getting error while running tblgen


I am trying to build a new backend target. First I wanted to start with just one intrinsic function. So I followed (I think I did) the
guidelines about doing. For some reason I am getting this error message from the tblgen:

                    "In SU_routeInstr: Type inference contradiction found in node imm!"

Here is my

def Imm8Arg: Operand<i8> {
        let PrintMethod = "print Imm8Arg";

// Intrinsic functions
// int_stella_su_route
let neverHasSideEffects = 1 in {

def SU_routeInstr: MyInst<(outs),(ins Imm8Arg:$inport, Imm8Arg:$outline),
                                [(int_my_su_route imm:$inport, imm:$outline)]>;

My intrinsic function is defined this way in the .td file that describes my target intrinsics:

def int_my_su_route :
              Intrinsic<[llvm_void_ty], [llvm_i8_ty,
                         llvm_i8_ty], [IntrNoMem]>;

This int_my_su_route function is of "void" type and it should take two byte-size immediate arguments.

Can anyone point me to my error?

Thanks a lot.

-- Fima