diff -u b/src/Query/Query.php b/src/Query/Query.php
--- b/src/Query/Query.php
+++ b/src/Query/Query.php
@@ -239,7 +239,7 @@
   /**
    * {@inheritdoc}
    */
-  public function setFulltextFields($fields = NULL) {
+  public function setFulltextFields(array $fields = NULL) {
     $fulltext_fields = $this->index->getFulltextFields();
     foreach (array_diff($fields, $fulltext_fields) as $field_id) {
       throw new SearchApiException(SafeMarkup::format('Trying to search on field @field which is no indexed fulltext field.', array('@field' => $field_id)));
@@ -319,11 +319,6 @@
     if (!$this->preExecuteRan) {
       $this->preExecuteRan = TRUE;
 
-      // Add fulltext fields, unless set
-      if ($this->fields === NULL) {
-        $this->fields = $this->index->getFulltextFields();
-      }
-
       // Preprocess query.
       $this->index->preprocessSearchQuery($this);
 
diff -u b/src/Query/QueryInterface.php b/src/Query/QueryInterface.php
--- b/src/Query/QueryInterface.php
+++ b/src/Query/QueryInterface.php
@@ -85,7 +85,7 @@
    * @throws \Drupal\search_api\SearchApiException
    *   Thrown if one of the fields isn't a fulltext field.
    */
-  public function setFulltextFields($fields = NULL);
+  public function setFulltextFields(array $fields = NULL);
 
   /**
    * Adds a subfilter to this query's filter.
@@ -230,9 +230,9 @@
   /**
    * Retrieves the fulltext fields that will be searched for the search keys.
    *
-   * @return NULL|string[]
+   * @return string[]|null
    *   An array containing the fields that should be searched for the search
-   *   keys or null.
+   *   keys or NULL if the method is called before QueryInterface::preExecute().
    *
    * @see fields()
    */