Getting unique values by excel formula

Dear all, I am using below formula for getting unique values, it is working fine except getting one Zero, below the formula & unique values example.

B2{=IFERROR(INDEX($A$2:$A$20, MATCH(0, COUNTIF($B1:B$1, $A$2:$A$20), 0)),"")}

CELL A   CELL B
a              a
a              b
a              c
b              0
c
c

here CELL B formula output is ok except Zero . I don't want to get Zero in the CELL B.