diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index a425f4f..0dd3ca1 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -812,7 +812,7 @@ protected function assertThemeOutput($callback, array $variables = array(), $exp
     /** @var \Drupal\Core\Render\RendererInterface $renderer */
     $renderer = \Drupal::service('renderer');
 
-    $output = $renderer->executeInRenderContext(new RenderContext(), function() use ($callback, $variables) {
+    $output = (string) $renderer->executeInRenderContext(new RenderContext(), function() use ($callback, $variables) {
       return \Drupal::theme()->render($callback, $variables);
     });
     $this->verbose(
diff --git a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
index c83b9c4..a62a862 100644
--- a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
@@ -1214,6 +1214,7 @@ public function isValueEmpty($value, $empty_zero, $no_skip_empty = TRUE) {
    */
   public function renderText($alter) {
     $value = $this->last_render;
+    $value_is_safe = SafeMarkup::isSafe($value);
 
     if (!empty($alter['alter_text']) && $alter['text'] !== '') {
       $tokens = $this->getRenderTokens($alter);
@@ -1239,6 +1240,9 @@ public function renderText($alter) {
     if ($alter['phase'] == static::RENDER_TEXT_PHASE_EMPTY && $no_rewrite_for_empty) {
       // If we got here then $alter contains the value of "No results text"
       // and so there is nothing left to do.
+      if ($value_is_safe) {
+        $value = SafeString::create($value);
+      }
       return $value;
     }
 
@@ -1275,6 +1279,11 @@ public function renderText($alter) {
     if (!empty($alter['nl2br'])) {
       $value = nl2br($value);
     }
+
+    // Preserve whether or not the string is safe.
+    if ($value_is_safe) {
+      $value = SafeString::create($value . $suffix);
+    }
     $this->last_render_text = $value;
 
     if (!empty($alter['make_link']) && (!empty($alter['path']) || !empty($alter['url']))) {
@@ -1284,7 +1293,13 @@ public function renderText($alter) {
       $value = $this->renderAsLink($alter, $value, $tokens);
     }
 
-    return $value . $suffix;
+    // Preserve whether or not the string is safe.
+    if ($value_is_safe) {
+      return SafeString::create($value . $suffix);
+    }
+    else {
+      return $value . $suffix;
+    }
   }
 
   /**