Dapper List Parameter rejected by Postgres

I have a List<Guid> (also tried it with a plain array) that I would like to use in a query. Dapper generates the SQL and substitutes the parameters for numbered params correctly. However, PG complains about the syntax at the list parameter. No further details are given. Copying the generated SQL and substituting the numbers with actual values (adding parens to the list param), the query works.

Here's the offending clause in the generated SQL:

WHERE (
    names.tenant = $1
    AND scan_results.tenant = $1
    AND scan_results.scan IN $2
)

The original statement string for Dapper is this:

WHERE (
    file_names.tenant = @tenantId
    AND scan_results.tenant = @tenantId
    AND scan_results.scan IN @validScans
)

The query is executed using con.Query like this:

con.Query(stmt, new
{
    tenantId = tenant.Id,
    validScans = tenant.LatestScans // .ToArray()
});

The exact Postgres error is:

ERROR:  syntax error at or near "$2" at character ...