$in needs an array

[relunctance]

                    <h2>Call Stack</h2>
                    <ol>
                        <li>in 
                            <a class="toggle" title="black/src/app/vendor/topthink/think-mongo/src/Connection.php line 291">Connection.php line 291</a>
                        </li>
                        <li>
            at 
                            <abbr title="MongoDB\Driver\Manager">Manager</abbr>->executeQuery('mdb_mid_m2.m2_mid', 
                            <em>object</em>(
                            <abbr title="MongoDB\Driver\Query">Query</abbr>), 
                            <em>null</em>) in 
                            <a class="toggle" title="/black/src/app/vendor/topthink/think-mongo/src/Connection.php line 291">Connection.php line 291</a>
                        </li>
                        <li>
            at 
                            <abbr title="think\mongo\Connection">Connection</abbr>->query('mdb_mid_m2.m2_mid', 
                            <em>object</em>(
                            <abbr title="MongoDB\Driver\Query">Query</abbr>), 
                            <em>null</em>, 
                            <em>false</em>, ['root' => 'array', 'document' => 'array']) in 
                            <a class="toggle" title="/black/src/app/vendor/topthink/think-mongo/src/Query.php line 254">Query.php line 254</a>
                        </li>
                        <li>
            at 
                            <abbr title="think\mongo\Query">Query</abbr>->query('m2_mid', 
                            <em>object</em>(
                            <abbr title="MongoDB\Driver\Query">Query</abbr>), 
                            <em>null</em>, 
                            <em>false</em>, ['root' => 'array', 'document' => 'array']) in 
                            <a class="toggle" title="/black/src/app/vendor/topthink/think-mongo/src/Query.php line 1746">Query.php line 1746</a>
                        </li>
                        <li>
            at 
                            <abbr title="think\mongo\Query">Query</abbr>->select() in 
                            <a class="toggle" title="/black/src/app/application/index/mongo/Midm2.php line 62">Midm2.php line 62</a>
                        </li>
                        <li>