reverted:
--- b/src/Entity/Index.php
+++ a/src/Entity/Index.php
@@ -582,11 +582,6 @@
    */
   public function getFields($only_indexed = TRUE) {
     $this->computeFields();
-
-    // Add fulltext fields, unless set.
-    if ($this->fields === NULL) {
-      $this->fields = $this->index->getFulltextFields();
-    }
     $only_indexed = $only_indexed ? 1 : 0;
     return $this->fields[$only_indexed]['fields'];
   }
diff -u b/src/Plugin/views/query/SearchApiQuery.php b/src/Plugin/views/query/SearchApiQuery.php
--- b/src/Plugin/views/query/SearchApiQuery.php
+++ b/src/Plugin/views/query/SearchApiQuery.php
@@ -568,7 +568,7 @@
    *
    * @see \Drupal\search_api\Query\QueryInterface::setFulltextFields()
    */
-  public function setFulltextFields($fields = array()) {
+  public function setFulltextFields($fields = NULL) {
     if (!$this->shouldAbort()) {
       $this->query->setFulltextFields($fields);
     }
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(array $fields = array()) {
+  public function setFulltextFields($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)));
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(array $fields = array());
+  public function setFulltextFields($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 string[]
+   * @return NULL|string[]
    *   An array containing the fields that should be searched for the search
-   *   keys.
+   *   keys or null.
    *
    * @see fields()
    */