"ABAP Basics" Book - arithmetic expression error in the sample code

Question

Hi all,
I have just started learning ABAP on mini SAP system (NW7.01) by following "ABAP Basics" book.
This question might be a FAQ, but I have spent a few days searching a right answer without success.
I appreciate if you could help me on this.
On the page 162, there is a function module sample, which contains the line as following:
e_amount = i_km * '0.3'.
Here l_km (import) is type of i, and e_amount (export) is type of f.
Though I do not think of any problem with this line, it throws an arithmetic expression error when executed directly for testing as well as when called from a program.
When I tried a similar thing in a program, it was fine.
However within a function module it throws this error.
Thanks in advance.
Terry

Answer 1

Like I said before, I do not think any problem about the code itself.
I suspect some environmental things?
I should have mentioned SAP mini system NW7.01 is running on Vista Business?
To be specifc, I receive this message:
Arithmetic operations are only expected for operands that can be converted to numbers.
(numeric operands). - (numeric operands). - - - -
with the following function module:
[code]
FUNCTION ZPTB00_CALCULATE_TRAVEL_EXPENS.
""Local Interface:
*"  IMPORTING
*"     VALUE(I_KM) TYPE REF TO  I
*"  EXPORTING
*"     VALUE(E_AMOUNT) TYPE REF TO  F
*"  EXCEPTIONS
*"      FAILED
e_amount = i_km * '0.3'.
IF sy-subrc <> 0.
  RAISE failed.
ENDIF.
ENDFUNCTION.
[/code]