Node-postgres parameterized query for INSERT INTO if not exists

I have a working parameterized query for an insert statement but I now want to add in a 'WHERE NOT EXISTS' clause. The working insert looks like this:

pgClient.query("INSERT INTO social_posts (username, user_image, message, image_url, post_id, post_url, location, network) VALUES ($1,$2,$3,$4,$5,$6,$7,$8)", postArray, 
    function(err, result) {...}

What I'd like to implement is:

pgClient.query("INSERT INTO social_posts(username, user_image, message, image_url, post_id, post_url, location, network) SELECT ($1,$2,$3,$4,$5,$6,$7,$8) WHERE NOT EXISTS (SELECT 1 FROM social_posts WHERE post_id = $9)", postArray,
    function(err, result) {...}

In this case the $9 would actually be equal to postArray[4].

I've tried pushing the value into the array again, that didn't work:

error running query { [error: operator does not exist: character varying = bigint]
name: 'error',
length: 207,
severity: 'ERROR',
code: '42883',
detail: undefined,
hint: 'No operator matches the given name and argument type(s). You might need to add explicit type casts.',
position: '198',
internalPosition: undefined,
internalQuery: undefined,
where: undefined,
schema: undefined,
table: undefined,
column: undefined,
dataType: undefined,
constraint: undefined,
file: 'parse_oper.c',
line: '722',
routine: 'op_error' }

I tried interpolating the value, that didn't work:

error running query { [error: there is no parameter $1]
name: 'error',
length: 88,
severity: 'ERROR',
code: '42P02',
detail: undefined,
hint: undefined,
position: '114',
internalPosition: undefined,
internalQuery: undefined,
where: undefined,
schema: undefined,
table: undefined,
column: undefined,
dataType: undefined,
constraint: undefined,
file: 'parse_expr.c',
line: '823',
routine: 'transformParamRef' }

Does anyone have any ideas? Thanks in advance!