diff --git a/src/main/php/com/mongodb/io/Commands.class.php b/src/main/php/com/mongodb/io/Commands.class.php
index 787e06a..c68e61e 100755
--- a/src/main/php/com/mongodb/io/Commands.class.php
+++ b/src/main/php/com/mongodb/io/Commands.class.php
@@ -23,7 +23,7 @@ private function __construct($proto, $conn) {
   /** Creates an instance for reading */
   public static function reading(Protocol $proto): self {
     return new self($proto, $proto->establish(
-      $proto->candidates($proto->readPreference['mode']),
+      $proto->candidates($proto->readPreference),
       'reading with '.$proto->readPreference['mode']
     ));
   }
diff --git a/src/main/php/com/mongodb/io/Protocol.class.php b/src/main/php/com/mongodb/io/Protocol.class.php
index a23ff0a..3a05ea9 100755
--- a/src/main/php/com/mongodb/io/Protocol.class.php
+++ b/src/main/php/com/mongodb/io/Protocol.class.php
@@ -141,20 +141,20 @@ public function connect() {
    *
    * @see    https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#read-preference
    * @see    https://docs.mongodb.com/manual/core/read-preference-mechanics/
-   * @param  string $rp
+   * @param  [:var] $rp
    * @return string[]
    * @throws lang.IllegalArgumentException
    */
   public function candidates($rp) {
-    if ('primary' === $rp) {
+    if ('primary' === $rp['mode']) {
       return [$this->nodes['primary']];
-    } else if ('secondary' === $rp) {
+    } else if ('secondary' === $rp['mode']) {
       return $this->nodes['secondary'];
-    } else if ('primaryPreferred' === $rp) {
+    } else if ('primaryPreferred' === $rp['mode']) {
       return array_merge([$this->nodes['primary']], $this->nodes['secondary']);
-    } else if ('secondaryPreferred' === $rp) {
+    } else if ('secondaryPreferred' === $rp['mode']) {
       return array_merge($this->nodes['secondary'], [$this->nodes['primary']]);
-    } else if ('nearest' === $rp) {  // Prefer to stay on already open connections
+    } else if ('nearest' === $rp['mode']) {  // Prefer to stay on already open connections
       $connected= null;
       foreach ($this->conn as $id => $conn) {
         if (null === $conn->server) continue;
@@ -229,7 +229,7 @@ public function read(array $options, $sections) {
     }
 
     $rp= $sections['$readPreference'] ?? $this->readPreference;
-    return $this->establish($this->candidates($rp['mode']), 'reading with '.$rp['mode'])
+    return $this->establish($this->candidates($rp), 'reading with '.$rp['mode'])
       ->message($sections, $rp)
     ;
   }