diff --git a/src/sessions.ts b/src/sessions.ts
index 0c5f23012f..237065634f 100644
--- a/src/sessions.ts
+++ b/src/sessions.ts
@@ -291,7 +291,7 @@ class ClientSession extends EventEmitter {
   /**
    * This is here to ensure that ClientSession is never serialized to BSON.
    */
-  toBSON(): void {
+  toBSON(): never {
     throw new Error('ClientSession cannot be serialized to BSON.');
   }