title: $search mustNot using synonyms
body: <p>I have an Atlas Search index that covers multiple fields and also uses a couple synonyms. I am getting an error when I try to use synonyms in the mustNot. Is this error telling me I am not able to use synonyms in the mustNot part of the compound? Any suggestions on how to work around this?</p>
  <p>MongoServerError: Remote error from mongot :: caused by :: Cannot call docFreq() when needsStats=false</p>
  <pre><code> {
                      '$search': {
                          &quot;index&quot;: &quot;default&quot;,
                          &quot;compound&quot;: {
                            &quot;must&quot;: [{
                              &quot;text&quot; : {
                                &quot;query&quot;: &quot;NY&quot;,
                                &quot;path&quot; : &quot;state&quot;,
                                &quot;synonyms&quot;: &quot;synonyms_state&quot;
                              }
                              },
                            ],
                            &quot;mustNot&quot;: [{
                              &quot;text&quot; : {
                                &quot;query&quot;: 'NYC',
                                &quot;path&quot; : &quot;city&quot;,
                                &quot;synonyms&quot;: &quot;synonyms_city&quot;
                              }
                            }]
                        }
                      }
                  }
  </code></pre>