Reporting Services :: Unrecognized Identifier Error In Custom Code?

I've made a Report porting from SS2008 to SS2008.

In a textbox I use a:

Number -> Custom Format

like these:

=Code.FormatString(Fields!NumDecRpt.Value)

but gives me the "Unrecognized identifier" red squiggle.

My FormatString function (in the rdl file) is this:

Public Shared Function FormatString(ByVal Decimals as object) As String
  dim format = "N0"
  if not (Decimals is nothing or Decimals is System.DBNull.Value) then
    dim decs as integer
    decs = Convert.ToInt32(Decimals)
    if decs>0 then
      format = "N"+decs.ToString()
    end if
  end if
  return format
End Function

What's wrong with this?