=LOOKUP(0,A2:A6,B2:B6) looks up 0 in column A, and returns an error because 0 is less than the smallest value : LOOKUP « Lookup Reference functions « Microsoft Office Excel 2007 Tutorial

1. Microsoft Office Excel 2007 Tutorial
2. Lookup Reference functions
3. LOOKUP

Input the formula: =LOOKUP(0,A2:A6,B2:B6). Press Enter to get the result.

19.10.LOOKUP
	19.10.1.	LOOKUP looks up values from a one-row or one-column range or an array
	19.10.2.	=LOOKUP(5,A2:A6,B2:B6) looks up 5 in column A, matches the next smallest value (3), and returns the value from column B that's in the same row
	19.10.3.	=LOOKUP(6.66,A2:A6,B2:B6) looks up 6.66 in column A, matches the next smallest value, and returns the value from column B that's in the same row
	19.10.4.	=LOOKUP(0,A2:A6,B2:B6) looks up 0 in column A, and returns an error because 0 is less than the smallest value
	19.10.5.	LOOKUP(lookup_value,array)
	19.10.6.	=LOOKUP("abcd",{"a",1;"b",2;"c",3})
	19.10.7.	=LOOKUP(A3,{3,6,7,8,9},{"F","D","C","B","A"})
	19.10.8.	=LOOKUP(A4,{0,70,73,77,80,83,87,90,93,97},{"F","D+","C-","C","C+","B-","B","B+","A-","A","A+"})