XML : Oracle method dispatch on NULL SELF argument is disallowed

I have a function under a package. I am passing some xml data to that function. But I'm getting this error. Could anybody explain it? I am explaining what I want to do in my code. I want to input all fields except the tsih_rate field. It will be calculated and filled in an automated way.

  FUNCTION save_settings_insurance_health(employeeData NVARCHAR2) RETURN CLOB IS    ret CLOB;  xmlData XMLType;  v_code NUMBER;  v_errm VARCHAR2(500);  input_rate number(7,2);    BEGIN    xmlData:=XMLType(employeeData);    INSERT INTO TBL_SETTINGS_INSURANCE_HEALTH SELECT temp1.* FROM XMLTABLE('/employees/employee/insurance/health'                                    PASSING xmlData                                    COLUMNS tsih_class NVARCHAR2(10) PATH 'tsih_class',                                            tsih_rate NUMBER(7,2) PATH 'tsih_rate' default null,                                            tsih_total NUMBER(7,2) PATH 'tsih_total',                                            tsih_eff_date_from DATE PATH 'tsih_eff_date_from',                                            tsih_eff_date_to DATE PATH 'tsih_eff_date_to',                                            tsih_created_on DATE PATH 'tsih_created_on',                                            tsih_created_by NVARCHAR2(10) PATH 'tsih_created_by')temp1;  ret:=to_char(sql%rowcount);  input_rate:=to_number(xmlData.extract('/employees/insurance/health/tsih_total/text()').getstringval())/12;  INSERT INTO TBL_SETTINGS_INSURANCE_HEALTH (tsih_rate) VALUES (input_rate);  COMMIT;    RETURN '<result><status affectedRow='||ret||'>success</status></result>';  DBMS_OUTPUT.PUT_LINE(ret);  EXCEPTION  WHEN OTHERS THEN  v_code := SQLCODE;  v_errm := SUBSTR(SQLERRM, 1 , 500);  DBMS_OUTPUT.PUT_LINE('Error code ' || v_code || ': ' || v_errm);  RETURN '<result><status>Error'||v_errm||'</status></result>';      END save_settings_insurance_health;      

My input pl/sql is as following:

  declare     result clob;  begin     -- Call the function     result := PMIS.SAVE_SETTINGS_INSURANCE_HEALTH('<employees>                        <employee>                          <insurance>                            <health>                              <tsih_class>teacher</tsih_class>                              <tsih_total>1200</tsih_total>                              <tsih_eff_date_from>2016-1-16</tsih_eff_date_from>                              <tsih_eff_date_to></tsih_eff_date_to>                              <tsih_created_on>2016-1-16</tsih_created_on>                              <tsih_created_by>Wahid</tsih_created_by>                            </health>                          </insurance>                        </employee>                    </employees>');      DBMS_OUTPUT.PUT_LINE(result);  end;