array – How to utilise multiple values from a single shortcode attribute?

i.e (test somevalues=1,2,3,4,5)

First time poster and alsoa php newb, I’m trying to add a feature to a plugin but I seem to keep triggering a debug notice from wordpress.

$somearray = shortcode_atts((
               'somevalues' => null,
    ), $atts);
$somearray('somevalues') = array_map( 'trim', str_getcsv( $somearray('somevalues'), ',' ) );

The above does work, but it results in a debug Notice: wpdb::prepare was called incorrectly.

Is there a way to achieve the same without triggering the notice? It doesn’t have to be csv, I just need it to return a string of somevalues, rather than returning an int. I guess what I want it to act like is as if it was written like (test somevalues=one,two,three,four,five)

Any suggestions on how to get around this?
Many thanks