diff --git a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
index d621897..613f88f 100644
--- a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
+++ b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
@@ -411,7 +411,7 @@ public function getDefaultValue(ContentEntityInterface $entity) {
    * @return $this
    */
   public function setDefaultValueCallback($callback) {
-    if (!is_string($callback)) {
+    if (isset($callback) && !is_string($callback)) {
       throw new \InvalidArgumentException('Default value callback must be a string, like "function_name" or "ClassName::methodName"');
     }
     $this->definition['default_value_callback'] = $callback;
diff --git a/core/lib/Drupal/Core/Field/FieldConfigBase.php b/core/lib/Drupal/Core/Field/FieldConfigBase.php
index 2548085..e0764cd 100644
--- a/core/lib/Drupal/Core/Field/FieldConfigBase.php
+++ b/core/lib/Drupal/Core/Field/FieldConfigBase.php
@@ -334,8 +334,8 @@ public function isRequired() {
    */
   public function getDefaultValue(ContentEntityInterface $entity) {
     // Allow custom default values function.
-    if ($function = $this->default_value_callback) {
-      $value = call_user_func($function, $entity, $this);
+    if ($callback = $this->default_value_callback) {
+      $value = call_user_func($callback, $entity, $this);
     }
     else {
       $value = $this->default_value;
diff --git a/core/modules/node/src/Tests/NodeFieldOverridesTest.php b/core/modules/node/src/Tests/NodeFieldOverridesTest.php
index 8ff6bde..8765353 100644
--- a/core/modules/node/src/Tests/NodeFieldOverridesTest.php
+++ b/core/modules/node/src/Tests/NodeFieldOverridesTest.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file
  * Contains \Drupal\node\Tests\NodeFieldOverridesTest.
diff --git a/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php b/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
index 440b997..98fd22e 100644
--- a/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
@@ -283,6 +283,16 @@ public function testInvalidDefaultValueCallback() {
   }
 
   /**
+   * Tests NULL default value callbacks.
+   *
+   * @covers ::setDefaultValueCallback
+   */
+  public function testNullDefaultValueCallback() {
+    $definition = BaseFieldDefinition::create($this->fieldType);
+    $definition->setDefaultValueCallback(NULL);
+  }
+
+  /**
    * Provides a Mock base field default value callback.
    *
    * @param \Drupal\Core\Entity\EntityInterface $entity