Different behaviour between interactive and not interactive usage

Hello,

I am trying to use Io under Ubuntu 10.04 
and have a strange behaviour:



% cat operator.io 
OperatorTable addOperator (":",1)
Object : := method (a, self print; a println)
1 : 3
% io operator.io 
1nil
% io < operator.io 
Io 20110905
Io> OperatorTable addOperator (":",1)
==> OperatorTable_0x8704dc0:
Operators
  0   ? @ @@
  1   ** :
  2   % * /
  3   + -
  4   << >>
  5   < <= > >=
  6   != ==
  7   &
  8   ^
  9   |
  10  && and
  11  or ||
  12  ..
  13  %= &= *= += -= /= <<= >>= ^= |=
  14  return

Assign Operators
  ::= newSlot
  :=  setSlot
  =   updateSlot

To add a new operator: OperatorTable addOperator("+", 4) and implement the + message.
To add a new assign operator: OperatorTable addAssignOperator("=", "updateSlot") and implement the updateSlot message.

Io> Object : := method (a, self print; a println)
==> method(a, 
    self print; a println
)
Io> 1 : 3
13
==> 3
Io> 
% 


So it seems that the interpreter sees the argument for operator : only in interactive mode. 
Could that be a bug?

Thanks in advance