Excel Error

#NULL!

Two ranges that don't intersect were combined with a space.

What #NULL! means

#NULL! happens when you use the intersection operator (a space) between ranges that don't actually overlap, or miss a comma between arguments.

Fix the operator

Use a comma for union (A1:A5,B1:B5) or a colon for a range (A1:B5).

What does the space operator mean?

It's the intersection operator — it returns cells common to both ranges, and #NULL! if there are none.