diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index 230a53eaf..1250f72d7 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -19,6 +19,8 @@ jobs:
           - '8.1'
           - '8.2'
           - '8.3'
+          - '8.4'
+          - '8.5'
 
     name: using PHP ${{ matrix.php-versions }}
     steps:
diff --git a/phpunit.xml b/phpunit.xml
index 4f4420bf5..92e12bb2f 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -8,6 +8,8 @@
     beStrictAboutChangesToGlobalState="true"
     beStrictAboutOutputDuringTests="true"
     beStrictAboutTestsThatDoNotTestAnything="true"
+    displayDetailsOnAllIssues="true"
+    failOnAllIssues="true"
 >
   <testsuites>
     <testsuite name="all">
diff --git a/src/JsonMapper.php b/src/JsonMapper.php
index 25534d0a6..937d971be 100644
--- a/src/JsonMapper.php
+++ b/src/JsonMapper.php
@@ -351,7 +351,9 @@ public function map($json, $object)
             $refDeserializePostMethod = $rc->getMethod(
                 $this->postMappingMethod
             );
-            $refDeserializePostMethod->setAccessible(true);
+            if (\PHP_VERSION_ID < 80100) {
+                $refDeserializePostMethod->setAccessible(true);
+            }
             $refDeserializePostMethod->invoke(
                 $object, ...$this->postMappingMethodArguments
             );
@@ -694,7 +696,9 @@ protected function setProperty(
         $object, $accessor, $value
     ) {
         if (!$accessor->isPublic() && $this->bIgnoreVisibility) {
-            $accessor->setAccessible(true);
+            if (\PHP_VERSION_ID < 80100) {
+                $accessor->setAccessible(true);
+            }
         }
         if ($accessor instanceof ReflectionProperty) {
             $accessor->setValue($object, $value);
@@ -754,7 +758,7 @@ protected function createInstance(
      */
     protected function getMappedType($type, $jvalue = null)
     {
-        if (isset($this->classMap[$type])) {
+        if (isset($this->classMap[$type ?? ''])) {
             $target = $this->classMap[$type];
         } else if (is_string($type) && $type !== '' && $type[0] == '\\'
             && isset($this->classMap[substr($type, 1)])
diff --git a/tests/support/JsonMapperTest/Object.php b/tests/support/JsonMapperTest/Object.php
index 3021255eb..544b972cf 100644
--- a/tests/support/JsonMapperTest/Object.php
+++ b/tests/support/JsonMapperTest/Object.php
@@ -31,7 +31,7 @@ class JsonMapperTest_Object
 
     public $docblockNullableObject;
 
-    public function setNullableObject(JsonMapperTest_PlainObject $obj = null)
+    public function setNullableObject(?JsonMapperTest_PlainObject $obj = null)
     {
         $this->nullableObject = $obj;
     }
diff --git a/tests/support/JsonMapperTest/PrivateWithSetter.php b/tests/support/JsonMapperTest/PrivateWithSetter.php
index 5256e5156..fce0b6ee6 100644
--- a/tests/support/JsonMapperTest/PrivateWithSetter.php
+++ b/tests/support/JsonMapperTest/PrivateWithSetter.php
@@ -52,7 +52,7 @@ public function setPrivatePropertySetterWithoutDoc(int $privateProperty)
         return $this;
     }
 
-    public function setPrivatePropertyNullableSetterWithoutDoc(int $privateProperty = null)
+    public function setPrivatePropertyNullableSetterWithoutDoc(?int $privateProperty = null)
     {
         $this->privatePropertyNullableSetterWithoutDoc = $privateProperty;
         return $this;