Can not get dbms_datapump.data_remap to work

I have created a package with a procedure that is tring to use the Data Pump API. I am using Oracle 11.1.0.6.0 Standard Edition and the SCOTT Schema.  I have no idea what I am doing wrong

I am getting an ORA-39001: invalid argument value error at this call:

dbms_datapump.data_remap (handle => dph,
                          name => 'COLUMN_FUNCTION',
                          table_name => mask_tab,
                          column => mask_col,
                          function => 'ORA_ADMIN.DATA_MASKER_UTIL.mask_number_random',
                          schema => 'SCOTT');

My Funtion is:

FUNCTION mask_varchar2 (data_to_mask VARCHAR2) RETURN VARCHAR2 IS      
    ret_val VARCHAR2(4000);       
BEGIN       
    ret_val := dbms_random.string('U', length(data_to_mask));