diff --git a/.github/workflows/continuous-integration.yml b/.github/workflows/continuous-integration.yml
index a5fa3ad4..3830e9fd 100644
--- a/.github/workflows/continuous-integration.yml
+++ b/.github/workflows/continuous-integration.yml
@@ -20,6 +20,7 @@ jobs:
           - "8.1"
           - "8.2"
           - "8.3"
+          - "8.4"
 
     steps:
       - name: "Checkout"
@@ -151,6 +152,7 @@ jobs:
           - "8.1"
           - "8.2"
           - "8.3"
+          - "8.4"
 
     steps:
       - name: "Checkout"
@@ -205,7 +207,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php-versions: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
+        php-versions: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Checkout
diff --git a/phpunit-windows.xml b/phpunit-windows.xml
index 0efef1d0..ac9f211c 100644
--- a/phpunit-windows.xml
+++ b/phpunit-windows.xml
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- https://phpunit.de/manual/current/en/appendixes.configuration.html -->
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.0/phpunit.xsd" backupGlobals="false" bootstrap="vendor\autoload.php" colors="true" processIsolation="false" stopOnFailure="false" cacheDirectory=".phpunit.cache" backupStaticProperties="false">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.0/phpunit.xsd" backupGlobals="false" bootstrap="vendor\autoload.php" colors="true" processIsolation="false" stopOnFailure="false" cacheDirectory=".phpunit.cache" backupStaticProperties="false" displayDetailsOnTestsThatTriggerWarnings="true">
   <coverage>
     <include>
       <directory>src</directory>
diff --git a/phpunit.xml b/phpunit.xml
index 3feaa53c..3eb1ce55 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- https://phpunit.de/manual/current/en/appendixes.configuration.html -->
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.0/phpunit.xsd" backupGlobals="false" bootstrap="vendor/autoload.php" colors="true" processIsolation="false" stopOnFailure="false" cacheDirectory=".phpunit.cache" backupStaticProperties="false">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.0/phpunit.xsd" backupGlobals="false" bootstrap="vendor/autoload.php" colors="true" processIsolation="false" stopOnFailure="false" cacheDirectory=".phpunit.cache" backupStaticProperties="false" displayDetailsOnTestsThatTriggerWarnings="true">
   <coverage>
     <include>
       <directory>src</directory>
diff --git a/src/Smalot/PdfParser/Font.php b/src/Smalot/PdfParser/Font.php
index af2d0f9f..56dee588 100644
--- a/src/Smalot/PdfParser/Font.php
+++ b/src/Smalot/PdfParser/Font.php
@@ -287,8 +287,13 @@ public function calculateTextWidth(string $text, ?array &$missing = null): ?floa
         // Usually, Widths key is set in $details array, but if it isn't use an empty array instead.
         $widths = $details['Widths'] ?? [];
 
-        // Widths array is zero indexed but table is not. We must map them based on FirstChar and LastChar
-        $width_map = array_flip(range($details['FirstChar'], $details['LastChar']));
+        /*
+         * Widths array is zero indexed but table is not. We must map them based on FirstChar and LastChar
+         *
+         * Note: Without the change you would see warnings in PHP 8.4 because the values of FirstChar or LastChar
+         *       can be null sometimes.
+         */
+        $width_map = array_flip(range((int) $details['FirstChar'], (int) $details['LastChar']));
 
         $width = null;
         $missing = [];