diff --git a/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/DedupeBase.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/DedupeBase.php
index 47fcee1..2900f1d 100644
--- a/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/DedupeBase.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/DedupeBase.php
@@ -35,11 +35,11 @@ public function transform($value, MigrateExecutable $migrate_executable, Row $ro
     }
     $start = isset($this->configuration['start']) ? $this->configuration['start'] : 0;
     if (!is_int($start)) {
-      throw new MigrateException('Input start position should be an integer.');
+      throw new MigrateException('The start position configuration key should be an integer. Omit this key to capture from the beginning of the string.');
     }
     $length = isset($this->configuration['length']) ? $this->configuration['length'] : NULL;
     if (!is_null($length) && !is_int($length)) {
-      throw new MigrateException('Input character length should be an integer.');
+      throw new MigrateException('The character length configuration key should be an integer. Omit this key to capture the entire string.');
     }
     // Use optional start or length to return a portion of deduplicated value.
     $new_value = Unicode::substr($new_value, $start, $length);
diff --git a/core/modules/migrate/tests/Drupal/migrate/Tests/process/DedupeEntityTest.php b/core/modules/migrate/tests/Drupal/migrate/Tests/process/DedupeEntityTest.php
index 7d1bef0..9c987c3 100644
--- a/core/modules/migrate/tests/Drupal/migrate/Tests/process/DedupeEntityTest.php
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/DedupeEntityTest.php
@@ -101,25 +101,32 @@ public function testDedupe($count, $postfix = '', $start = NULL, $length = NULL)
   }
 
   /**
-   * Tests that invalid substring start and length options throw exceptions.
+   * Tests that invalid start position throws an exception.
    */
-  public function testDedupeEntityInvalidOptions() {
+  public function testDedupeEntityInvalidStart() {
     $configuration = array(
       'entity_type' => 'test_entity_type',
       'field' => 'test_field',
       'start' => 'foobar',
     );
-    // Test an invalid substring start position.
     $plugin = new DedupeEntity($configuration, 'dedupe_entity', array(), $this->getMigration(), $this->entityQueryFactory);
     $this->entityQueryExpects(0);
-    $this->setExpectedException('Drupal\migrate\MigrateException', 'Input start position should be an integer.');
+    $this->setExpectedException('Drupal\migrate\MigrateException', 'The start position configuration key should be an integer. Omit this key to capture from the beginning of the string.');
     $plugin->transform('test_start', $this->migrateExecutable, $this->row, 'testproperty');
+  }
 
-    // Test an invalid substring character length.
-    $configuration['length'] = 'foobar';
+  /**
+   * Tests that invalid length option throws an exception.
+   */
+  public function testDedupeEntityInvalidLength() {
+    $configuration = array(
+      'entity_type' => 'test_entity_type',
+      'field' => 'test_field',
+      'length' => 'foobar',
+    );
     $plugin = new DedupeEntity($configuration, 'dedupe_entity', array(), $this->getMigration(), $this->entityQueryFactory);
     $this->entityQueryExpects(0);
-    $this->setExpectedException('Drupal\migrate\MigrateException', 'Input character length should be an integer.');
+    $this->setExpectedException('Drupal\migrate\MigrateException', 'The character length configuration key should be an integer. Omit this key to capture the entire string.');
     $plugin->transform('test_length', $this->migrateExecutable, $this->row, 'testproperty');
   }