diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index fa17d9c8de8..802c91711c8 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -1080,7 +1080,7 @@ function template_preprocess_file_link(&$variables) {
   // Set file classes to the options array.
   $variables['attributes'] = new Attribute($variables['attributes']);
   $variables['attributes']->addClass($classes);
-  $variables['file_size'] = format_size($file->getSize() ?? 0);
+  $variables['file_size'] = $file->getSize() !== NULL ? format_size($file->getSize()) : '';
 
   $variables['link'] = Link::fromTextAndUrl($link_text, $url->mergeOptions($options))->toRenderable();
 }
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php b/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php
index 3c0aa914fab..e94156a6d36 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php
@@ -39,7 +39,7 @@ class TableFormatter extends DescriptionAwareFileFormatterBase {
               ],
             ],
           ],
-          ['data' => format_size($file->getSize())],
+          ['data' => $file->getSize() !== NULL ? format_size($file->getSize()) : $this->t('Unknown')],
         ];
       }
 
diff --git a/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php b/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php
index 7b12a90dc8e..43dfa4fc238 100644
--- a/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php
+++ b/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php
@@ -232,6 +232,16 @@ class FileFieldDisplayTest extends FileFieldTestBase {
 
     $this->drupalGet('node/' . $nid);
     $this->assertSession()->elementTextContains('xpath', '//a[@href="' . $node->{$field_name}->entity->createFileUrl() . '"]', $description);
+
+    // Test that null file size is rendered as "Unknown".
+    $nonexistent_file = File::create([
+      'uri' => 'temporary://' . $this->randomMachineName() . '.txt',
+    ]);
+    $nonexistent_file->save();
+    $node->set($field_name, $nonexistent_file->id());
+    $node->save();
+    $this->drupalGet('node/' . $nid);
+    $this->assertSession()->elementTextEquals('xpath', '//a[@href="' . $node->{$field_name}->entity->createFileUrl() . '"]/../../../td[2]', 'Unknown');
   }
 
 }