Compile error in Oracle Reports Developer.

I'm getting an Error 201 in one of my Program Units.  The code is as follows:

function CF_revisionFormula return VARCHAR2 is
  rev_flag varchar2(1) := ' ';
begin
  select 'R' into rev_flag
    from operation_notes
  where :op_id = operation_notes.op_id and
            on_type_cd = 'T' and
            rownum = 1;
  return (rev_flag);
RETURN NULL; exception
  when others then return (' ');
end;

The error I receive is:  Error 201 at line 5, column 10 identifier 'OPERATION_NOTES' must be declared
                                  Error 0 at line 4, column 3  SQL Statement ignored.

I have other Program Units for other reports coded in the same mannor and I don't get an error on them.  Where should I start looking for the problem?