Form Select causing 500 error

I’m having an issue with an insert form. If I submit the form using text fields and just input a number for id fields it adds fine. If I make those selects with the value being the id from the referenced table I get this error -

serverconnectForm.js:249 POST http://www.triptakers.travel/dmxConnect/api/Forms/add_tripbit.php 500 (Internal Server Error)