#REF! notification while refering to another worksheet on excel

While trying to link 2 worksheets on excel with one another, I keep getting a #REF! output on my cells on worksheet 1 that are refering to the other worksheet (2). This only happens when I close the worksheet which is being referred at (2). What to do about this problem?

Sidenote: when I open worksheet 2 again, the #ref! output disappears and the formula works perfectly fine again, so I don't really understand how this problem occurs(??)