diff --git a/src/Dev/Tasks/FileMigrationHelper.php b/src/Dev/Tasks/FileMigrationHelper.php
index c02252c7..2c258962 100644
--- a/src/Dev/Tasks/FileMigrationHelper.php
+++ b/src/Dev/Tasks/FileMigrationHelper.php
@@ -210,7 +210,7 @@ protected function ss3Migration($base)
      * If `$initialStrategy` is not suitable for a migration, we return null.
      * We're overriding the helpers to avoid doing unnecessary checks.
      * @param FileResolutionStrategy $initialStrategy
-     * @return int|FileIDHelperResolutionStrategy
+     * @return int|FileIDHelperResolutionStrategy|null
      */
     private function buildSS3MigrationStrategy(FileResolutionStrategy $initialStrategy)
     {
diff --git a/src/Dev/TestAssetStore.php b/src/Dev/TestAssetStore.php
index bdd53667..e464215b 100644
--- a/src/Dev/TestAssetStore.php
+++ b/src/Dev/TestAssetStore.php
@@ -105,7 +105,7 @@ public static function activate($basedir)
     /**
      * Get absolute path to basedir
      *
-     * @return string
+     * @return string|null
      */
     public static function base_path()
     {
diff --git a/src/File.php b/src/File.php
index ae411b8c..7f116300 100644
--- a/src/File.php
+++ b/src/File.php
@@ -609,7 +609,7 @@ public function getStatusTitle()
      * Possible group values are: "audio", "mov", "zip", "image".
      *
      * @param string $ext Extension to check
-     * @return string
+     * @return string|false
      */
     public static function get_app_category($ext)
     {
@@ -732,7 +732,7 @@ protected function onBeforeWrite()
             // and any file extensions removed.
             $this->setField(
                 'Title',
-                str_replace(['-','_'], ' ', preg_replace('/\.[^.]+$/', '', $name))
+                str_replace(['-','_'], ' ', preg_replace('/\.[^.]+$/', '', $name) ?: '')
             );
         }
 
@@ -872,7 +872,7 @@ protected function getNameGenerator($filename)
     /**
      * Gets the URL of this file
      *
-     * @return string
+     * @return string|null
      */
     public function getAbsoluteURL()
     {
@@ -888,7 +888,7 @@ public function getAbsoluteURL()
      *
      * @uses Director::baseURL()
      * @param bool $grant Ensures that the url for any protected assets is granted for the current user.
-     * @return string
+     * @return string|null
      */
     public function getURL($grant = true)
     {
@@ -902,7 +902,7 @@ public function getURL($grant = true)
      * Get URL, but without resampling.
      *
      * @param bool $grant Ensures that the url for any protected assets is granted for the current user.
-     * @return string
+     * @return string|null
      */
     public function getSourceURL($grant = true)
     {
@@ -1019,7 +1019,7 @@ public static function get_file_extension($filename)
      * Given an extension, determine the icon that should be used
      *
      * @param string $extension
-     * @return string Icon filename relative to base url
+     * @return string|null Icon filename relative to base url
      */
     public static function get_icon_for_extension($extension)
     {
diff --git a/src/FilenameParsing/FileIDHelperResolutionStrategy.php b/src/FilenameParsing/FileIDHelperResolutionStrategy.php
index f13a36d5..4e8e33f6 100644
--- a/src/FilenameParsing/FileIDHelperResolutionStrategy.php
+++ b/src/FilenameParsing/FileIDHelperResolutionStrategy.php
@@ -523,7 +523,7 @@ public function stripVariant($fileID)
      *
      * @param ParsedFileID $parsedFileID
      * @param FileIDHelper $helper
-     * @return ParsedFileID
+     * @return ParsedFileID|null
      */
     private function stripVariantFromParsedFileID(ParsedFileID $parsedFileID, FileIDHelper $helper)
     {
diff --git a/src/FilenameParsing/LegacyFileIDHelper.php b/src/FilenameParsing/LegacyFileIDHelper.php
index 18380924..7097b1cd 100644
--- a/src/FilenameParsing/LegacyFileIDHelper.php
+++ b/src/FilenameParsing/LegacyFileIDHelper.php
@@ -133,7 +133,7 @@ public function parseFileID($fileID)
         return new ParsedFileID(
             $filename,
             '',
-            isset($matches['variant']) ? str_replace('/', '_', $matches['variant']) : '',
+            isset($matches['variant']) ? str_replace('/', '_', $matches['variant'] ?: '') : '',
             $fileID
         );
     }
diff --git a/src/Folder.php b/src/Folder.php
index d8cd036f..bbff876f 100644
--- a/src/Folder.php
+++ b/src/Folder.php
@@ -238,7 +238,7 @@ public function getURL($grant = true)
     /**
      * Folders do not have public URLs
      *
-     * @return string
+     * @return string|null
      */
     public function getAbsoluteURL()
     {
diff --git a/src/ImageManipulation.php b/src/ImageManipulation.php
index ab2efc67..1e2254b7 100644
--- a/src/ImageManipulation.php
+++ b/src/ImageManipulation.php
@@ -1024,7 +1024,7 @@ public function variantParts($variantName)
 
         // Regex needs to be case insensitive since allMethodNames() is all lowercased
         $regex = '#^(?<format>(' . implode('|', $methods) . '))(?<encodedargs>(.*))#i';
-        preg_match($regex, $variantName, $matches);
+        preg_match($regex, (string) $variantName, $matches);
 
         if (!$matches) {
             throw new InvalidArgumentException('Invalid variant name: ' . $variantName);
diff --git a/src/InterventionBackend.php b/src/InterventionBackend.php
index 8a01913b..6b51f416 100644
--- a/src/InterventionBackend.php
+++ b/src/InterventionBackend.php
@@ -214,7 +214,7 @@ public function loadFromContainer(AssetContainer $assetContainer)
      * Get the currently assigned image resource, or generates one if not yet assigned.
      * Note: This method may return null if error
      *
-     * @return InterventionImage
+     * @return InterventionImage|null
      */
     public function getImageResource()
     {
@@ -351,7 +351,7 @@ public function setImageResource($image)
      * @param string $hash Hash of original file, if storing a variant.
      * @param string $variant Name of variant, if storing a variant.
      * @param array $config Write options. {@see AssetStore}
-     * @return array Tuple associative array (Filename, Hash, Variant) Unless storing a variant, the hash
+     * @return array|null Tuple associative array (Filename, Hash, Variant) Unless storing a variant, the hash
      * will be calculated from the given data.
      * @throws BadMethodCallException If image isn't valid
      */
diff --git a/src/Shortcodes/FileShortcodeProvider.php b/src/Shortcodes/FileShortcodeProvider.php
index 6d20f06c..ffa0587c 100644
--- a/src/Shortcodes/FileShortcodeProvider.php
+++ b/src/Shortcodes/FileShortcodeProvider.php
@@ -70,7 +70,7 @@ public static function get_shortcodes()
      * @param string $shortcode Name of shortcode used to register this handler
      * @param array $extra Extra arguments
      *
-     * @return string Result of the handled shortcode
+     * @return string|null Result of the handled shortcode
      */
     public static function handle_shortcode($arguments, $content, $parser, $shortcode, $extra = [])
     {
@@ -202,7 +202,7 @@ public static function find_shortcode_record($args, &$errorCode = null)
      *
      * @param int $errorCode HTTP Error value
      *
-     * @return File|SiteTree File or SiteTree object to use for the given error
+     * @return File|SiteTree|null File or SiteTree object to use for the given error
      */
     protected static function find_error_record($errorCode)
     {
diff --git a/src/Storage/DBFile.php b/src/Storage/DBFile.php
index 0f826cad..b1a493e5 100644
--- a/src/Storage/DBFile.php
+++ b/src/Storage/DBFile.php
@@ -151,7 +151,7 @@ public function getTag()
     /**
      * Determine the template to render as on the frontend
      *
-     * @return string Name of template
+     * @return string|null Name of template
      */
     public function getFrontendTemplate()
     {
@@ -173,7 +173,7 @@ public function getFrontendTemplate()
     /**
      * Get trailing part of filename
      *
-     * @return string
+     * @return string|null
      */
     public function getBasename()
     {
@@ -186,7 +186,7 @@ public function getBasename()
     /**
      * Get file extension
      *
-     * @return string
+     * @return string|null
      */
     public function getExtension()
     {
@@ -318,7 +318,7 @@ public function getSourceURL($grant = true)
     /**
      * Get the absolute URL to this resource
      *
-     * @return string
+     * @return string|null
      */
     public function getAbsoluteURL()
     {
diff --git a/src/Storage/DefaultAssetNameGenerator.php b/src/Storage/DefaultAssetNameGenerator.php
index f2ebe347..fd70a692 100644
--- a/src/Storage/DefaultAssetNameGenerator.php
+++ b/src/Storage/DefaultAssetNameGenerator.php
@@ -100,7 +100,7 @@ public function __construct($filename)
             $this->name = $matches['name'];
             // Check if number is padded
             if (strpos($matches['version'], '0') === 0) {
-                $this->padding = strlen($matches['version']);
+                $this->padding = strlen((string) $matches['version']);
             }
         } else {
             $this->first = 1;
diff --git a/src/Storage/ProtectedFileController.php b/src/Storage/ProtectedFileController.php
index d3f0480c..083b3241 100644
--- a/src/Storage/ProtectedFileController.php
+++ b/src/Storage/ProtectedFileController.php
@@ -75,7 +75,7 @@ public function handleFile(HTTPRequest $request)
     public function isValidFilename($filename)
     {
         // Block hidden files
-        return !preg_match('#(^|[\\\\/])\\..*#', $filename);
+        return !preg_match('#(^|[\\\\/])\\..*#', (string) $filename);
     }
 
     /**
diff --git a/tests/php/RedirectFileControllerTest.php b/tests/php/RedirectFileControllerTest.php
index 14ec74e3..f00e512d 100644
--- a/tests/php/RedirectFileControllerTest.php
+++ b/tests/php/RedirectFileControllerTest.php
@@ -530,7 +530,7 @@ protected function assertResponse($code, $body, $location, HTTPResponse $respons
      */
     protected function normaliseUrl($path)
     {
-        return str_replace('RedirectFileControllerTest/', '', $path);
+        return str_replace('RedirectFileControllerTest/', '', $path ?: '');
     }
 
     public function get($url, $session = null, $headers = null, $cookies = null)