无法将变量从控制器传递到模型以处理codeigniter中的多个上载字段

I have three file inputs in a form (each of them with different name: self_pic, passport_pic, and sic_pic). in a controller that handles the inputs, i assign three names of uploaded image to each different variables and i pass the variables into a function that i already defined in a model, so that i can save the variables that contain names of uploaded image in db. it works fine for two variables and not for one variable. I keep getting error that the variable is undefined.I have three file inputs in a form (each of the