diff --git a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
index b127a229..a96bdd4 100644
--- a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
+++ b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
@@ -438,16 +438,21 @@ public function setDefaultValueCallback($callback) {
    *     the first item.
    *   - a numerically indexed array of items, each item being a property/value
    *     array.
+   *   - a non-numerically indexed array, in which case the array is assumed to
+   *     be a property/value array and used as the first item
    *   - NULL or array() for no default value.
    *
    * @return $this
    */
   public function setDefaultValue($value) {
-    if (!is_array($value)) {
-      $value = array(array($this->getMainPropertyName() => $value));
-    }
-    elseif (!is_numeric(array_keys($value)[0])) {
-      $value = array(0 => $value);
+    // Unless the value is NULL or an empty array, we may need to transform it.
+    if (!(is_null($value) || (is_array($value) && empty($value)))) {
+      if (!is_array($value)) {
+        $value = array(array($this->getMainPropertyName() => $value));
+      }
+      elseif (!is_numeric(array_keys($value)[0])) {
+        $value = array(0 => $value);
+      }
     }
     $this->definition['default_value'] = $value;
     return $this;
diff --git a/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php b/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
index ae71170..2b1b3ce 100644
--- a/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/BaseFieldDefinitionTest.php
@@ -189,6 +189,14 @@ public function testFieldDefaultValue() {
     // Set default value with an indexed array.
     $definition->setDefaultValue($expected_default_value);
     $this->assertEquals($expected_default_value, $definition->getDefaultValue($entity));
+
+    // Set default value with an empty array.
+    $definition->setDefaultValue(array());
+    $this->assertEquals(array(), $definition->getDefaultValue($entity));
+
+    // Set default value with NULL.
+    $definition->setDefaultValue(NULL);
+    $this->assertEquals(NULL, $definition->getDefaultValue($entity));
   }
 
   /**