TypeError: Cannot read property 'toUpperCase' of null

I’m gonna need an expert here to explain this to me. I submit my code and it says: Oops, try again and on the console it says: TypeError: Cannot read property ‘toUpperCase’ of null, which is all greek to me. here is all of my code that has .toUpperCase in it:

var user = prompt(“What will you do? ANSWERS: Take the glinting thing, sneak up on the wriggling thing, OR open the door.”).toUpperCase();

var p0001 = prompt(“Do you want to wait?”).toUpperCase();

var p0002 = prompt(“Is there anything else you want to do before going down?”).toUpperCase();

var p003 = prompt(“What will you do? ANSWERS: Take the glinting thing, OR go down the stairs.”).toUpperCase();

var p004 = prompt(“Do you get ye flask?”).toUpperCase();

var p005 = prompt(“Do you open the door”).toUpperCase();

var p006 = prompt(“Do you get ye flask?”).toUpperCase();

var p007 = prompt(“Do you open the door”).toUpperCase();

can anyone help? I’m not quite sure what’s wrong.