diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index b1e51d3..12fa9fd 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -29,7 +29,6 @@ jobs:
       fail-fast: false
       matrix:
         php_version:
-          - "7.4"
           - "8.0"
           - "8.1.0RC6"
         include:
diff --git a/.github/workflows/updated.yml b/.github/workflows/updated.yml
index 2bdae8c..5fa3409 100644
--- a/.github/workflows/updated.yml
+++ b/.github/workflows/updated.yml
@@ -29,7 +29,6 @@ jobs:
       fail-fast: false
       matrix:
         php_version:
-          - "7.4"
           - "8.0"
           - "8.1.0RC6"
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diff --git a/composer.json b/composer.json
index 9605aad..78d615e 100644
--- a/composer.json
+++ b/composer.json
@@ -23,16 +23,16 @@
     }
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-    "php": ">=7.4"
+    "php": ">=8.0"
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   "require-dev": {
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-    "phpmd/phpmd": "2.10.*",
+    "phpmd/phpmd": "2.11.*",
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-    "psr/cache": "1.0.*",
+    "psr/cache": "3.0.*",
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     "squizlabs/php_codesniffer": "3.6.*"
@@ -57,8 +57,7 @@
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-      "dev-main": "1.5.x-dev",
-      "dev-release": "1.4.x-dev"
+      "dev-main": "1.5.x-dev"
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diff --git a/composer.lock b/composer.lock
index 1d8e21f..27494bf 100644
--- a/composer.lock
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@@ -4,7 +4,7 @@
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+    "content-hash": "c49d4f52f1bee5eed7d256bbb1282b98",
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@@ -326,16 +326,16 @@
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+            "version": "2.10.2",
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-                "reference": "30452fdabb3dfca89f4bf977abc44adc5391e062"
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-                "reference": "30452fdabb3dfca89f4bf977abc44adc5391e062",
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-                "source": "https://github.com/pdepend/pdepend/tree/2.10.1"
+                "source": "https://github.com/pdepend/pdepend/tree/2.10.2"
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@@ -379,7 +379,7 @@
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+            "time": "2021-11-16T20:05:32+00:00"
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@@ -711,22 +711,22 @@
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-            "version": "2.10.2",
+            "version": "2.11.0",
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                 "url": "https://github.com/phpmd/phpmd.git",
-                "reference": "1bc74db7cf834662d83abebae265be11bb2eec3a"
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-                "reference": "1bc74db7cf834662d83abebae265be11bb2eec3a",
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-                "pdepend/pdepend": "^2.10.0",
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@@ -782,7 +782,7 @@
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-                "source": "https://github.com/phpmd/phpmd/tree/2.10.2"
+                "source": "https://github.com/phpmd/phpmd/tree/2.11.0"
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@@ -790,7 +790,7 @@
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-            "time": "2021-07-22T09:56:23+00:00"
+            "time": "2021-11-29T14:05:52+00:00"
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@@ -910,16 +910,16 @@
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+            "version": "9.2.9",
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-                "reference": "cf04e88a2e3c56fc1a65488afd493325b4c1bc3e"
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-                "reference": "cf04e88a2e3c56fc1a65488afd493325b4c1bc3e",
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@@ -975,7 +975,7 @@
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-                "source": "https://github.com/sebastianbergmann/php-code-coverage/tree/9.2.8"
+                "source": "https://github.com/sebastianbergmann/php-code-coverage/tree/9.2.9"
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@@ -983,7 +983,7 @@
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-            "time": "2021-10-30T08:01:38+00:00"
+            "time": "2021-11-19T15:21:02+00:00"
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@@ -1331,20 +1331,20 @@
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-            "version": "1.0.1",
+            "version": "3.0.0",
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                 "url": "https://github.com/php-fig/cache.git",
-                "reference": "d11b50ad223250cf17b86e38383413f5a6764bf8"
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-                "php": ">=5.3.0"
+                "php": ">=8.0.0"
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@@ -1364,7 +1364,7 @@
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                     "name": "PHP-FIG",
-                    "homepage": "http://www.php-fig.org/"
+                    "homepage": "https://www.php-fig.org/"
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@@ -1374,9 +1374,9 @@
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-                "source": "https://github.com/php-fig/cache/tree/master"
+                "source": "https://github.com/php-fig/cache/tree/3.0.0"
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-            "time": "2016-08-06T20:24:11+00:00"
+            "time": "2021-02-03T23:26:27+00:00"
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@@ -1428,30 +1428,30 @@
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+            "version": "3.0.0",
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+                "php": ">=8.0.0"
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+                    "dev-master": "3.x-dev"
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-                    "Psr\\Log\\": "Psr/Log/"
+                    "Psr\\Log\\": "src"
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@@ -1472,9 +1472,9 @@
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-                "source": "https://github.com/php-fig/log/tree/1.1.4"
+                "source": "https://github.com/php-fig/log/tree/3.0.0"
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-            "time": "2021-05-03T11:20:27+00:00"
+            "time": "2021-07-14T16:46:02+00:00"
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@@ -1905,16 +1905,16 @@
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-            "version": "4.0.3",
+            "version": "4.0.4",
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@@ -1963,14 +1963,14 @@
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-            "homepage": "http://www.github.com/sebastianbergmann/exporter",
+            "homepage": "https://www.github.com/sebastianbergmann/exporter",
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-                "source": "https://github.com/sebastianbergmann/exporter/tree/4.0.3"
+                "source": "https://github.com/sebastianbergmann/exporter/tree/4.0.4"
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@@ -1978,7 +1978,7 @@
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@@ -2620,22 +2620,22 @@
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+                "symfony/deprecation-contracts": "^2.1|^3",
+                "symfony/filesystem": "^4.4|^5.0|^6.0",
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@@ -2644,11 +2644,11 @@
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-                "symfony/messenger": "^4.4|^5.0",
-                "symfony/service-contracts": "^1.1|^2",
-                "symfony/yaml": "^4.4|^5.0"
+                "symfony/event-dispatcher": "^4.4|^5.0|^6.0",
+                "symfony/finder": "^4.4|^5.0|^6.0",
+                "symfony/messenger": "^4.4|^5.0|^6.0",
+                "symfony/service-contracts": "^1.1|^2|^3",
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@@ -2679,7 +2679,7 @@
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+                "source": "https://github.com/symfony/config/tree/v5.4.0"
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@@ -2695,27 +2695,28 @@
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+            "version": "v5.4.0",
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@@ -2730,9 +2731,9 @@
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-                "symfony/config": "^5.3",
-                "symfony/expression-language": "^4.4|^5.0",
-                "symfony/yaml": "^4.4|^5.0"
+                "symfony/config": "^5.3|^6.0",
+                "symfony/expression-language": "^4.4|^5.0|^6.0",
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@@ -2767,7 +2768,7 @@
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@@ -2834,7 +2835,7 @@
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@@ -2850,25 +2851,26 @@
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@@ -2897,7 +2899,7 @@
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+                "source": "https://github.com/symfony/filesystem/tree/v5.4.0"
             },
             "funding": [
                 {
@@ -2913,7 +2915,7 @@
                     "type": "tidelift"
                 }
             ],
-            "time": "2021-07-21T12:40:44+00:00"
+            "time": "2021-10-28T13:39:27+00:00"
         },
         {
             "name": "symfony/polyfill-ctype",
@@ -2994,6 +2996,86 @@
             ],
             "time": "2021-02-19T12:13:01+00:00"
         },
+        {
+            "name": "symfony/polyfill-mbstring",
+            "version": "v1.23.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/polyfill-mbstring.git",
+                "reference": "9174a3d80210dca8daa7f31fec659150bbeabfc6"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/polyfill-mbstring/zipball/9174a3d80210dca8daa7f31fec659150bbeabfc6",
+                "reference": "9174a3d80210dca8daa7f31fec659150bbeabfc6",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=7.1"
+            },
+            "suggest": {
+                "ext-mbstring": "For best performance"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-main": "1.23-dev"
+                },
+                "thanks": {
+                    "name": "symfony/polyfill",
+                    "url": "https://github.com/symfony/polyfill"
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "Symfony\\Polyfill\\Mbstring\\": ""
+                },
+                "files": [
+                    "bootstrap.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Symfony polyfill for the Mbstring extension",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "compatibility",
+                "mbstring",
+                "polyfill",
+                "portable",
+                "shim"
+            ],
+            "support": {
+                "source": "https://github.com/symfony/polyfill-mbstring/tree/v1.23.1"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2021-05-27T12:26:48+00:00"
+        },
         {
             "name": "symfony/polyfill-php80",
             "version": "v1.23.1",
@@ -3158,22 +3240,25 @@
         },
         {
             "name": "symfony/service-contracts",
-            "version": "v2.4.0",
+            "version": "v2.4.1",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/service-contracts.git",
-                "reference": "f040a30e04b57fbcc9c6cbcf4dbaa96bd318b9bb"
+                "reference": "d664541b99d6fb0247ec5ff32e87238582236204"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/symfony/service-contracts/zipball/f040a30e04b57fbcc9c6cbcf4dbaa96bd318b9bb",
-                "reference": "f040a30e04b57fbcc9c6cbcf4dbaa96bd318b9bb",
+                "url": "https://api.github.com/repos/symfony/service-contracts/zipball/d664541b99d6fb0247ec5ff32e87238582236204",
+                "reference": "d664541b99d6fb0247ec5ff32e87238582236204",
                 "shasum": ""
             },
             "require": {
                 "php": ">=7.2.5",
                 "psr/container": "^1.1"
             },
+            "conflict": {
+                "ext-psr": "<1.1|>=2"
+            },
             "suggest": {
                 "symfony/service-implementation": ""
             },
@@ -3217,7 +3302,7 @@
                 "standards"
             ],
             "support": {
-                "source": "https://github.com/symfony/service-contracts/tree/v2.4.0"
+                "source": "https://github.com/symfony/service-contracts/tree/v2.4.1"
             },
             "funding": [
                 {
@@ -3233,7 +3318,7 @@
                     "type": "tidelift"
                 }
             ],
-            "time": "2021-04-01T10:43:52+00:00"
+            "time": "2021-11-04T16:37:19+00:00"
         },
         {
             "name": "theseer/tokenizer",
@@ -3350,7 +3435,7 @@
     "prefer-stable": false,
     "prefer-lowest": false,
     "platform": {
-        "php": ">=7.4"
+        "php": ">=8.0"
     },
     "platform-dev": [],
     "plugin-api-version": "2.1.0"
diff --git a/docker-compose.yml b/docker-compose.yml
index c607597..553deb9 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -6,7 +6,7 @@ services:
     build:
       context: "."
       args:
-        PHP_VERSION: "${PHP_VERSION:-7.4}"
+        PHP_VERSION: "${PHP_VERSION:-8.0}"
     volumes:
     - ".:/app"
     user: "${UID:-1000}:${GID:-1000}"
diff --git a/src/Filter/IntToRoman.php b/src/Filter/IntToRoman.php
index 463dc40..dedade5 100644
--- a/src/Filter/IntToRoman.php
+++ b/src/Filter/IntToRoman.php
@@ -35,7 +35,7 @@ public function __construct(?Grammar $grammar = null)
     private function cache(int $value, string $result): void
     {
         if ($this->hasCache()) {
-            $item = $this->getCache()->getItem($value)->set($result);
+            $item = $this->getCache()->getItem((string) $value)->set($result);
             $this->getCache()->save($item);
         }
     }
@@ -52,8 +52,8 @@ public function filter(int $value): string
             throw new Exception(sprintf('Invalid integer: %d', $value), Exception::INVALID_INTEGER);
         }
 
-        if ($this->hasCache() && $this->getCache()->hasItem($value)) {
-            return $this->getCache()->getItem($value)->get();
+        if ($this->hasCache() && $this->getCache()->hasItem((string) $value)) {
+            return $this->getCache()->getItem((string) $value)->get();
         }
 
         $tokens = $this->getGrammar()->getTokens();