diff -u b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
--- b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
+++ b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
@@ -133,10 +133,10 @@
    *   (optional) How many rows the source plugin is expected to return.
    *   Defaults to count($expected_data). If set to a non-null, non-numeric
    *   value (like FALSE or 'nope'), the source plugin will not be counted.
-   * @param mixed $high_water
-   *   (optional) The value of the high water field.
    * @param array $configuration
    *   (optional) Configuration for the source plugin.
+   * @param mixed $high_water
+   *   (optional) The value of the high water field.
    *
    * @dataProvider providerSource
    */
@@ -158,8 +158,8 @@
       $expected_count = count($expected_data);
     }
     // If an expected count was given, assert it only if the plugin is
-    // countable.
-    if (is_numeric($expected_count)) {
+    // countable and there is no high water mark.
+    if (is_numeric($expected_count) && !isset($high_water)) {
       $this->assertInstanceOf('\Countable', $plugin);
       $this->assertCount($expected_count, $plugin);
     }
@@ -185,6 +185,10 @@
         }
       }
     }
+
+    if (isset($high_water)) {
+      $this->assertSame($expected_count, $i);
+    }
   }
 
 }