title: Detecting blank cells through vlookup
body: <p>ISBLANK detects blank cells, but returns FALSE for cells with formulae. I need to detect blank cells that has been looked up through VLOOKUP.</p>
  
  <p>Example, A1 .. F7:</p>
  
  <pre>
  1  foo     =isblank(b1)  =vlookup(a1,a$1:b$9,2)  =isblank(d1)
  2  a       =isblank(b2)  =vlookup(a2,a$1:b$9,2)  =isblank(d2)
  3  0       =isblank(b3)  =vlookup(a3,a$1:b$9,2)  =isblank(d3)
  4  42      =isblank(b4)  =vlookup(a4,a$1:b$9,2)  =isblank(d4)
  5  ="bar"  =isblank(b5)  =vlookup(a5,a$1:b$9,2)  =isblank(d5)
  6  =""     =isblank(b6)  =vlookup(a6,a$1:b$9,2)  =isblank(d6)
  7          =isblank(b7)  =vlookup(a7,a$1:b$9,2)  =isblank(d7)
  </pre>
  
  <p>Results:</p>
  
  <pre>
  1  foo     FALSE         foo                     FALSE
  2  a       FALSE         a                       FALSE
  3  0       FALSE         0                       FALSE
  4  42      FALSE         42                      FALSE
  5  ="bar"  FALSE         bar                     FALSE
  6  =""     FALSE                                 FALSE
  7          TRUE                                  FALSE
  </pre>
  
  <p>I tried a workaround with LEN, but that will return 1 for blank cells (blank evaluates numerically to 0 and LEN converts this to text before checking length). There is also the problem in row 6 where LEN (correctly) returns 0.</p>
  
  <p>Is there a way?</p>
  
  <p>I am primarily using openoffice and libreoffice, but this should also apply to excel.</p>