Insert Rows Based On Text Values?

I am an Oracle beginner and I am having some trouble with the following insert query.

I am inputting values into text boxes and then this is carried out as a trigger upon clicking a button.

INSERT INTO client VALUES(':student.txtclientid', ':student.txtclientname', ':student.clientaddress', 13564338);
INSERT INTO enrolment VALUES(':student.txtclientid', ':student.lstoccurrence', null, null);

The above text boxes are all working fine as I have viewed the values using the message command. My proplem is that if i leave the fields blank it inserts ':student.txtclientname' into the row, otherwise it returns "Could not insert record"