ERROR: index row requires 9984 bytes, maximum size is 8191

Hi all -

I have added a column to a table is which of datatype text. I am
trying to create an index

CREATE OR REPLACE FUNCTION reverse(varchar) RETURNS varchar AS $$
$reversed = reverse $_[0];
return $reversed;
$$ LANGUAGE plperlu IMMUTABLE;

CREATE INDEX rev_email ON users( (reverse(email) ) varchar_pattern_ops );

I am getting the following error ERROR: index row requires 9984
bytes, maximum size is 8191

my logs are not showing any thing else , but the error. Can you please help?

Regards