diff --git a/composer.json b/composer.json
index 9c2fdf9..c223113 100644
--- a/composer.json
+++ b/composer.json
@@ -22,7 +22,7 @@
     "source": "https://github.com/vanderlee/phpSyllable/"
   },
   "require": {
-    "php": ">=5.6",
+    "php": ">=7.1",
     "ext-json": "*",
     "ext-libxml": "*",
     "ext-mbstring": "*",
diff --git a/src/Syllable.php b/src/Syllable.php
index 0132910..02a71f4 100644
--- a/src/Syllable.php
+++ b/src/Syllable.php
@@ -162,7 +162,7 @@ public function getHyphen()
     /**
      * @param Cache $cache
      */
-    public function setCache(Cache $cache = null)
+    public function setCache(?Cache $cache = null)
     {
         $this->cache = $cache;
     }
@@ -598,8 +598,8 @@ private function parseHtmlText($html)
      */
     private function hyphenateHtmlDom(
         DOMNode $node,
-        DOMNodeList $excludeNodes = null,
-        DOMNodeList $includeNodes = null,
+        ?DOMNodeList $excludeNodes = null,
+        ?DOMNodeList $includeNodes = null,
         $split = true
     ) {
         if ($node->hasChildNodes()) {
diff --git a/tests/src/SyllableTest.php b/tests/src/SyllableTest.php
index 52ecc16..1781b76 100644
--- a/tests/src/SyllableTest.php
+++ b/tests/src/SyllableTest.php
@@ -171,8 +171,8 @@ public function testCache($cacheClass, $language, $cacheDirectory, $cacheFile, $
         $this->assertNotEmpty($this->object->getCache()->__get('patterns'));
         $this->assertGreaterThan(0, $this->object->getCache()->__get('max_pattern'));
         $this->assertNotEmpty($this->object->getCache()->__get('hyphenation'));
-        $this->assertInternalType('int', $this->object->getCache()->__get('left_min_hyphen'));
-        $this->assertInternalType('int', $this->object->getCache()->__get('right_min_hyphen'));
+        $this->assertIsInt($this->object->getCache()->__get('left_min_hyphen'));
+        $this->assertIsInt($this->object->getCache()->__get('right_min_hyphen'));
     }
 
     public function dataCacheVersionMatchesCacheFileVersionIsRelaxed()