how to use a function with two parameters in parameter sql statement

how can I set the second bind parameter to null.

I have tried select bolinf.XXPA_RPT_PROJNUM_PKG.get_selected_list(:$FLEX$.XXPA_PW_PROJECT_NUMBERS, nvl(:$FLEX$.XXPA_PW_PROJECTS_NUM, NULL)) from dual;

bolinf.XXPA_RPT_PROJNUM_PKG.get_selected_list(:$FLEX$.XXPA_PW_PROJECT_NUMBERS, :$FLEX$.XXPA_PW_PROJECTS_NUM DEFAULT NULL) from dual;

neither works for the function

bolinf.XXPA_RPT_PROJNUM_PKG.get_selected_list(:$FLEX$.XXPA_PW_PROJECT_NUMBERS, NULL) from dual this works but I need it to be a variable.

It works fine in sql developer but not in EBS R12