Create index on array element?

I guess one cannot create an index on an element of an array?

test=> create table test ( ordinal int[] );
CREATE TABLE
test=> create unique index x1 on test ( ordinal[1] );
ERROR: syntax error at or near ")" at character 45
LINE 1: create unique index x1 on test ( ordinal[1] );
^

Correct, or is my syntax wrong?