Embedded code didn't work

In the code tab of Report Properties window, I pasted the following VB code:

Public Function Divide(Numerator As Double, Denominator As Double)
 If Denominator = 0 Then
  Return 0
 Else
  Return Numerator/Denominator
 End If
End Function

But when I tried to reference the above function using "Code." in my expression, I saw an "Unrecognized Identifier" when I put my mouse onto this function in the expression edit window. Why? What I did is just copy the above function to the code tab and then start to reference it in the expression, what did I missed? Thanks for your opinion.