diff -u b/src/Form/SensorForm.php b/src/Form/SensorForm.php
--- b/src/Form/SensorForm.php
+++ b/src/Form/SensorForm.php
@@ -237,8 +237,7 @@
       $plugin = $sensor_config->getPlugin();
     }
 
-    if (!empty($plugin->getDefaultConfiguration())) {
-      $default_config = $plugin->getDefaultConfiguration();
+    if ($default_config = $plugin->getDefaultConfiguration()) {
       $form_state->setValue(['settings', 'table'], $default_config['table']);
       $form_state->setValue(['settings', 'aggregation', 'time_interval_field'], $default_config['time_interval_field']);
     }
diff -u b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php
--- b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php
+++ b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php
@@ -55,8 +55,8 @@
    */
   public function getDefaultConfiguration() {
     $default_config = [
-       'table' => 'watchdog',
-       'time_interval_field' => 'timestamp',
+      'table' => 'watchdog',
+      'time_interval_field' => 'timestamp',
      ];
     return $default_config;
   }
only in patch2:
unchanged:
--- a/src/Tests/MonitoringUITest.php
+++ b/src/Tests/MonitoringUITest.php
@@ -177,6 +177,16 @@ class MonitoringUITest extends MonitoringTestBase {
     $sensor_theme = SensorConfig::load('core_theme_default');
     $this->assertTrue($sensor_theme->status());
 
+    // Test the creation of a Watchdog sensor.
+    $this->drupalGet('admin/config/system/monitoring/sensors/add');
+    $this->drupalPostForm(NULL, array(
+      'label' => 'Watchdog Sensor',
+      'id' => 'watchdog_sensor',
+      'plugin_id' => 'watchdog_aggregator',
+    ), t('Select sensor'));
+
+    $this->drupalPostForm(NULL, array(), t('Save'));
+    $this->assertText(SafeMarkup::format('Sensor @label saved.', array('@label' => 'Watchdog Sensor')));
   }
 
   /**