diff -u b/core/modules/migrate/tests/src/Unit/process/DedupeEntityTest.php b/core/modules/migrate/tests/src/Unit/process/DedupeEntityTest.php
--- b/core/modules/migrate/tests/src/Unit/process/DedupeEntityTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/DedupeEntityTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\migrate\Unit\process;
 
+use Drupal\Core\Entity\Query\QueryInterface;
 use Drupal\migrate\Plugin\migrate\process\DedupeEntity;
 use Drupal\Component\Utility\Unicode;
 
@@ -176,13 +177,10 @@
     // 'test_vocab1'.
     $map = [];
     foreach (['forums', 'test_vocab', 'test_vocab1'] as $id) {
-      $query = $this->getMockBuilder('Drupal\Core\Entity\Query\QueryInterface')
-        ->disableOriginalConstructor()
-        ->getMock();
-      $query->expects($this->once())
-        ->method('execute')
-        ->will($this->returnValue($id === 'test_vocab1' ? [] : [$id]));
-      $map[] = ['test_field', $id, NULL, NULL, $query];
+      $query = $this->prophesize(QueryInterface::class);
+      $query->willBeConstructedWith([]);
+      $query->execute()->willReturn($id === 'test_vocab1' ? [] : [$id]);
+      $map[] = ['test_field', $id, NULL, NULL, $query->reveal()];
     }
     $this->entityQuery->expects($this->exactly(3))
       ->method('condition')