diff --git a/core/lib/Drupal/Core/Render/RenderCache.php b/core/lib/Drupal/Core/Render/RenderCache.php
index 99e2a9a..c33179a 100644
--- a/core/lib/Drupal/Core/Render/RenderCache.php
+++ b/core/lib/Drupal/Core/Render/RenderCache.php
@@ -340,7 +340,7 @@ public function getCacheableRenderArray(array $elements) {
       $data['#cache_properties'] = $elements['#cache_properties'];
       // Store whether any of the cache properties are safe strings.
       foreach (Element::properties(array_flip($elements['#cache_properties'])) as $cache_property) {
-        if (isset($elements[$cache_property]) && SafeMarkup::isSafe($elements[$cache_property])) {
+        if (isset($elements[$cache_property]) && !is_array($elements[$cache_property]) && SafeMarkup::isSafe($elements[$cache_property])) {
           $data['#safe_cache_properties'][] = $cache_property;
         }
       }
diff --git a/core/lib/Drupal/Core/Render/RenderCacheInterface.php b/core/lib/Drupal/Core/Render/RenderCacheInterface.php
index c0e580b..2d9552f 100644
--- a/core/lib/Drupal/Core/Render/RenderCacheInterface.php
+++ b/core/lib/Drupal/Core/Render/RenderCacheInterface.php
@@ -44,7 +44,10 @@ public function getCacheableRenderArray(array $elements);
    *
    * @return array|false
    *   A renderable array, with the original element and all its children pre-
-   *   rendered, or FALSE if no cached copy of the element is available.
+   *   rendered, or FALSE if no cached copy of the element is available. The
+   *   array contains a '#safe_cache_properties' key detailing any properties
+   *   that were safe strings when the render cache was set. The
+   *   '#safe_cache_properties' is removed once it is used by the render system.
    *
    * @see \Drupal\Core\Render\RendererInterface::render()
    * @see ::set()
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererTest.php b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
index 978ec7f..01857c9 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
@@ -695,6 +695,7 @@ public function testRenderCacheProperties(array $expected_results) {
       'child2' => ['#markup' => '2'],
       // Mark the value as safe.
       '#custom_property' => SafeMarkup::checkPlain('custom_value'),
+      '#custom_property_array' => ['custom value'],
     ];
 
     $this->renderer->renderRoot($element);
@@ -714,7 +715,9 @@ public function testRenderCacheProperties(array $expected_results) {
         $this->assertSame($data[$property], $original[$property]);
       }
     }
-    $this->assertArrayEquals(array_merge(['#markup'], Element::properties(array_filter($expected_results))), $data['#safe_cache_properties']);
+    // #custom_property_array can not be a safe_cache_property.
+    $safe_cache_properties = array_diff(array_merge(['#markup'], Element::properties(array_filter($expected_results))), ['#custom_property_array']);
+    $this->assertArrayEquals($safe_cache_properties, $data['#safe_cache_properties']);
   }
 
   /**
@@ -727,14 +730,15 @@ public function testRenderCacheProperties(array $expected_results) {
   public function providerTestRenderCacheProperties() {
     return [
       [[]],
-      [['child1' => 0, 'child2' => 0, '#custom_property' => 0]],
-      [['child1' => 0, 'child2' => 0, '#custom_property' => 1]],
-      [['child1' => 0, 'child2' => 1, '#custom_property' => 0]],
-      [['child1' => 0, 'child2' => 1, '#custom_property' => 1]],
-      [['child1' => 1, 'child2' => 0, '#custom_property' => 0]],
-      [['child1' => 1, 'child2' => 0, '#custom_property' => 1]],
-      [['child1' => 1, 'child2' => 1, '#custom_property' => 0]],
-      [['child1' => 1, 'child2' => 1, '#custom_property' => 1]],
+      [['child1' => 0, 'child2' => 0, '#custom_property' => 0, '#custom_property_array' => 0]],
+      [['child1' => 0, 'child2' => 0, '#custom_property' => 1, '#custom_property_array' => 0]],
+      [['child1' => 0, 'child2' => 1, '#custom_property' => 0, '#custom_property_array' => 0]],
+      [['child1' => 0, 'child2' => 1, '#custom_property' => 1, '#custom_property_array' => 0]],
+      [['child1' => 1, 'child2' => 0, '#custom_property' => 0, '#custom_property_array' => 0]],
+      [['child1' => 1, 'child2' => 0, '#custom_property' => 1, '#custom_property_array' => 0]],
+      [['child1' => 1, 'child2' => 1, '#custom_property' => 0, '#custom_property_array' => 0]],
+      [['child1' => 1, 'child2' => 1, '#custom_property' => 1, '#custom_property_array' => 0]],
+      [['child1' => 1, 'child2' => 1, '#custom_property' => 1, '#custom_property_array' => 1]],
     ];
   }