Oracle Message PLS-00440 - FORALL bulk IN-bind variables cannot be used here

ERROR	FORALL bulk IN-bind variables cannot be used here
CAUSE	A reference to a FORALL statements bulk IN-bind variable was found in a context in which it is not permitted. For example, such variables are not permitted in expressions in the USING clause of a dynamic SQL statement: forall j in 1..Users.Count() execute immediate insert into System_Users(Username) values : U using Upper(Users(j));
ACTION	In the case described above, move the expression from the USING clause into the SQL statement: forall j in 1..Users.Count() execute immediate insert into System_Users(Username) values Upper(: U) using Users(j);