diff --git a/lib/Element/Uri.php b/lib/Element/Uri.php
index 259dfa0..3f9308e 100644
--- a/lib/Element/Uri.php
+++ b/lib/Element/Uri.php
@@ -59,7 +59,7 @@ public function xmlSerialize(Xml\Writer $writer): void
     {
         $writer->text(
             resolve(
-                $writer->contextUri,
+                $writer->contextUri ?? '',
                 $this->value
             )
         );
diff --git a/lib/Service.php b/lib/Service.php
index ba1c443..1dd3427 100644
--- a/lib/Service.php
+++ b/lib/Service.php
@@ -241,6 +241,7 @@ public function write(string $rootElementName, $value, string $contextUri = null
     public function mapValueObject(string $elementName, string $className): void
     {
         list($namespace) = self::parseClarkNotation($elementName);
+        $namespace = $namespace ?? '';
 
         $this->elementMap[$elementName] = function (Reader $reader) use ($className, $namespace) {
             return \Sabre\Xml\Deserializer\valueObject($reader, $className, $namespace);
diff --git a/lib/Writer.php b/lib/Writer.php
index 9cc0001..88a298b 100644
--- a/lib/Writer.php
+++ b/lib/Writer.php
@@ -124,6 +124,7 @@ public function startElement($name): bool
         if ('{' === $name[0]) {
             list($namespace, $localName) =
                 Service::parseClarkNotation($name);
+            $namespace = $namespace ?? '';
 
             if (array_key_exists($namespace, $this->namespaceMap)) {
                 $result = $this->startElementNS(
@@ -134,7 +135,7 @@ public function startElement($name): bool
             } else {
                 // An empty namespace means it's the global namespace. This is
                 // allowed, but it mustn't get a prefix.
-                if ('' === $namespace || null === $namespace) {
+                if ('' === $namespace) {
                     $result = $this->startElement($localName);
                     $this->writeAttribute('xmlns', '');
                 } else {
@@ -238,6 +239,7 @@ public function writeAttribute($name, $value): bool
             $localName
         ) = Service::parseClarkNotation($name);
 
+        $namespace = $namespace ?? '';
         if (array_key_exists($namespace, $this->namespaceMap)) {
             // It's an attribute with a namespace we know
             return $this->writeAttribute(
diff --git a/phpstan.neon b/phpstan.neon
index 67b1aee..bb0d913 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -2,7 +2,7 @@ includes:
     - phpstan-baseline.neon
 
 parameters:
-  level: 7
+  level: 8
   phpVersion: 70430 # PHP 7.4.30
   paths:
     - lib