diff --git a/src/Header/Consumer/AbstractConsumerService.php b/src/Header/Consumer/AbstractConsumerService.php
index dd3ceb33..ee2bfe63 100644
--- a/src/Header/Consumer/AbstractConsumerService.php
+++ b/src/Header/Consumer/AbstractConsumerService.php
@@ -30,12 +30,12 @@ abstract class AbstractConsumerService implements IConsumerService
     protected LoggerInterface $logger;
 
     /**
-     * @var used to construct IHeaderPart objects
+     * @var HeaderPartFactory used to construct IHeaderPart objects
      */
     protected HeaderPartFactory $partFactory;
 
     /**
-     * @var the generated token split pattern on first run, so it doesn't
+     * @var ?string the generated token split pattern on first run, so it doesn't
      *      need to be regenerated every time.
      */
     private ?string $tokenSplitPattern = null;
diff --git a/src/Header/HeaderFactory.php b/src/Header/HeaderFactory.php
index fe75cbc0..f26f7fed 100644
--- a/src/Header/HeaderFactory.php
+++ b/src/Header/HeaderFactory.php
@@ -182,12 +182,13 @@ public function newInstance(string $name, string $value) : IHeader
     }
 
     /**
-     * Creates an IHeader instance for the passed header name and value, and
-     * returns it.
+     * Creates an IHeader instance for the passed header name and value using
+     * the passed IHeader class, and returns it.
      *
-     * @param $name The header name.
-     * @param $value The header value.
-     * @return The created header object.
+     * @param string $name The header name.
+     * @param string $value The header value.
+     * @param string $iHeaderClass The class to use for header creation
+     * @return IHeader The created header object.
      */
     public function newInstanceOf(string $name, string $value, string $iHeaderClass) : IHeader
     {
diff --git a/src/Header/IdHeader.php b/src/Header/IdHeader.php
index 328494e4..eae9c02b 100644
--- a/src/Header/IdHeader.php
+++ b/src/Header/IdHeader.php
@@ -9,6 +9,7 @@
 
 use ZBateson\MailMimeParser\Header\Part\MimeLiteralPartFactory;
 use ZBateson\MailMimeParser\Header\Consumer\IdBaseConsumerService;
+use ZBateson\MailMimeParser\Header\Part\CommentPart;
 
 /**
  * Represents a Content-ID, Message-ID, In-Reply-To or References header.
diff --git a/src/Header/Part/AddressGroupPart.php b/src/Header/Part/AddressGroupPart.php
index e567a1c8..b66151ea 100644
--- a/src/Header/Part/AddressGroupPart.php
+++ b/src/Header/Part/AddressGroupPart.php
@@ -80,7 +80,7 @@ protected function getErrorBagChildren() : array
 
     protected function validate() : void
     {
-        if (mb_strlen($this->value) === 0) {
+        if ($this->value === null || mb_strlen($this->value) === 0) {
             $this->addError('Address group doesn\'t have a name', LogLevel::ERROR);
         }
         if (empty($this->addresses)) {
diff --git a/src/Header/Part/Token.php b/src/Header/Part/Token.php
index 10d3e4b7..89e39cd1 100644
--- a/src/Header/Part/Token.php
+++ b/src/Header/Part/Token.php
@@ -38,7 +38,7 @@ public function __construct(MbWrapper $charsetConverter, $value)
      */
     public function isSpace() : bool
     {
-        return (\preg_match('/^\s+$/', $this->value) === 1);
+        return ($this->value !== null && \preg_match('/^\s+$/', $this->value) === 1);
     }
 
     /**
diff --git a/src/Header/ReceivedHeader.php b/src/Header/ReceivedHeader.php
index fd32344b..a4397bbf 100644
--- a/src/Header/ReceivedHeader.php
+++ b/src/Header/ReceivedHeader.php
@@ -114,7 +114,7 @@ public function getValue() : ?string
      * exists in this position is returned -- be it contains spaces, or invalid
      * characters, etc...
      *
-     * @return The 'FROM' name.
+     * @return ?string The 'FROM' name.
      */
     public function getFromName() : ?string
     {
@@ -131,7 +131,7 @@ public function getFromName() : ?string
      * not be valid.  More details on how the value is parsed and extracted can
      * be found in the class description for {@see ReceivedHeader}.
      *
-     * @return The 'FROM' hostname.
+     * @return ?string The 'FROM' hostname.
      */
     public function getFromHostname() : ?string
     {
@@ -148,7 +148,7 @@ public function getFromHostname() : ?string
      * not be valid.  More details on how the value is parsed and extracted can
      * be found in the class description for {@see ReceivedHeader}.
      *
-     * @return The 'FROM' address.
+     * @return ?string The 'FROM' address.
      */
     public function getFromAddress() : ?string
     {
@@ -165,7 +165,7 @@ public function getFromAddress() : ?string
      * exists in this position is returned -- be it contains spaces, or invalid
      * characters, etc...
      *
-     * @return The 'BY' name.
+     * @return ?string The 'BY' name.
      */
     public function getByName() : ?string
     {
@@ -182,7 +182,7 @@ public function getByName() : ?string
      * not be valid.  More details on how the value is parsed and extracted can
      * be found in the class description for {@see ReceivedHeader}.
      *
-     * @return The 'BY' hostname.
+     * @return ?string The 'BY' hostname.
      */
     public function getByHostname() : ?string
     {
@@ -199,7 +199,7 @@ public function getByHostname() : ?string
      * not be valid.  More details on how the value is parsed and extracted can
      * be found in the class description for {@see ReceivedHeader}.
      *
-     * @return The 'BY' address.
+     * @return ?string The 'BY' address.
      */
     public function getByAddress() : ?string
     {
diff --git a/src/Message.php b/src/Message.php
index cbd99547..0fc3cab0 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -54,7 +54,7 @@ public function __construct(
             $partChildrenContainer
         );
         if ($multipartHelper === null || $privacyHelper === null) {
-            $di = MailMimeParser::getDependencyContainer();
+            $di = MailMimeParser::getGlobalContainer();
             $multipartHelper = $di[\ZBateson\MailMimeParser\Message\Helper\MultipartHelper::class];
             $privacyHelper = $di[\ZBateson\MailMimeParser\Message\Helper\PrivacyHelper::class];
         }
diff --git a/src/Message/Helper/MultipartHelper.php b/src/Message/Helper/MultipartHelper.php
index 7104c2a2..e9fe10aa 100644
--- a/src/Message/Helper/MultipartHelper.php
+++ b/src/Message/Helper/MultipartHelper.php
@@ -13,6 +13,7 @@
 use ZBateson\MailMimeParser\Message\Factory\IUUEncodedPartFactory;
 use ZBateson\MailMimeParser\Message\IMessagePart;
 use ZBateson\MailMimeParser\Message\IMimePart;
+use ZBateson\MailMimeParser\Message\IMultiPart;
 use ZBateson\MailMimeParser\Message\PartFilter;
 
 /**
@@ -155,7 +156,7 @@ public function removeAllContentPartsFromAlternative(
             $alternativePart = $message->getPart(0, PartFilter::fromInlineContentType('multipart/alternative'));
         }
         $message->removePart($rmPart);
-        if ($alternativePart !== null) {
+        if ($alternativePart !== null && $alternativePart instanceof IMultiPart) {
             if ($alternativePart->getChildCount() === 1) {
                 $this->genericHelper->replacePart($message, $alternativePart, $alternativePart->getChild(0));
             } elseif ($alternativePart->getChildCount() === 0) {
@@ -385,7 +386,9 @@ public function setContentPartForMimeType(IMessage $message, string $mimeType, m
             $part = $this->createContentPartForMimeType($message, $mimeType, $charset);
         } else {
             $contentType = $part->getContentType();
-            $part->setRawHeader(HeaderConsts::CONTENT_TYPE, "$contentType;\r\n\tcharset=\"$charset\"");
+            if ($part instanceof IMimePart) {
+                $part->setRawHeader(HeaderConsts::CONTENT_TYPE, "$contentType;\r\n\tcharset=\"$charset\"");
+            }
         }
         $part->setContent($stringOrHandle);
         return $this;
diff --git a/src/Message/IMessagePart.php b/src/Message/IMessagePart.php
index f972dab7..fcfc8aca 100644
--- a/src/Message/IMessagePart.php
+++ b/src/Message/IMessagePart.php
@@ -73,7 +73,7 @@ public function getContentDisposition(?string $default = null) : ?string;
      * Returns the content transfer encoding used to encode the content on this
      * part, or the value of $default if not defined.
      *
-     * @param $default Optional default value to return if not
+     * @param ?string $default Optional default value to return if not
      *        applicable/defined
      * @return string|null the transfer encoding defined for the part.
      */
diff --git a/src/Message/IMultiPart.php b/src/Message/IMultiPart.php
index e647a09f..b8e76aec 100644
--- a/src/Message/IMultiPart.php
+++ b/src/Message/IMultiPart.php
@@ -249,10 +249,10 @@ public function getPartByContentId($contentId) : ?IMessagePart;
      * If the $position parameter is non-null, adds the part at the passed
      * position index, otherwise adds it as the last child.
      *
-     * @param IMessagePart $part The part to add.
+     * @param MessagePart $part The part to add.
      * @param int $position Optional insertion position 0-based index.
      */
-    public function addChild(IMessagePart $part, ?int $position = null) : static;
+    public function addChild(MessagePart $part, ?int $position = null) : static;
 
     /**
      * Removes the child part from this part and returns its previous position
diff --git a/src/Message/MultiPart.php b/src/Message/MultiPart.php
index 3cc76ce2..b1b19e3a 100644
--- a/src/Message/MultiPart.php
+++ b/src/Message/MultiPart.php
@@ -131,7 +131,7 @@ public function getPartByContentId($contentId) : ?IMessagePart
         });
     }
 
-    public function addChild(IMessagePart $part, ?int $position = null) : static
+    public function addChild(MessagePart $part, ?int $position = null) : static
     {
         if ($part !== $this) {
             $part->parent = $this;
diff --git a/src/Parser/Proxy/ParserMimePartProxy.php b/src/Parser/Proxy/ParserMimePartProxy.php
index 1e69c149..47b8392b 100644
--- a/src/Parser/Proxy/ParserMimePartProxy.php
+++ b/src/Parser/Proxy/ParserMimePartProxy.php
@@ -7,7 +7,7 @@
 
 namespace ZBateson\MailMimeParser\Parser\Proxy;
 
-use ZBateson\MailMimeParser\Header\IHeader;
+use ZBateson\MailMimeParser\Header\ParameterHeader;
 use ZBateson\MailMimeParser\Header\HeaderConsts;
 use ZBateson\MailMimeParser\Message\IMessagePart;
 use Psr\Log\LogLevel;
@@ -131,7 +131,7 @@ public function parseAll() : static
      * Returns a ParameterHeader representing the parsed Content-Type header for
      * this part.
      */
-    public function getContentType() : ?IHeader
+    public function getContentType() : ?ParameterHeader
     {
         return $this->getHeaderContainer()->get(HeaderConsts::CONTENT_TYPE);
     }
diff --git a/tests/MailMimeParser/Message/Helper/GenericHelperTest.php b/tests/MailMimeParser/Message/Helper/GenericHelperTest.php
index 26f6f091..6e7aa9a0 100644
--- a/tests/MailMimeParser/Message/Helper/GenericHelperTest.php
+++ b/tests/MailMimeParser/Message/Helper/GenericHelperTest.php
@@ -36,12 +36,18 @@ protected function setUp() : void
 
     private function newMockIMimePart() : \ZBateson\MailMimeParser\Message\IMimePart
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\Message\IMimePart::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message\MimePart::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newMockIMessage() : \ZBateson\MailMimeParser\IMessage
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\IMessage::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newGenericHelper() : GenericHelper
diff --git a/tests/MailMimeParser/Message/Helper/MultipartHelperTest.php b/tests/MailMimeParser/Message/Helper/MultipartHelperTest.php
index 30d88066..d55e755c 100644
--- a/tests/MailMimeParser/Message/Helper/MultipartHelperTest.php
+++ b/tests/MailMimeParser/Message/Helper/MultipartHelperTest.php
@@ -39,17 +39,26 @@ protected function setUp() : void
 
     private function newMockIMimePart() : \ZBateson\MailMimeParser\Message\IMimePart
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\Message\IMimePart::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message\MimePart::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newMockIUUEncodedPart() : \ZBateson\MailMimeParser\Message\IUUEncodedPart
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\Message\IUUEncodedPart::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message\UUEncodedPart::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newMockIMessage() : \ZBateson\MailMimeParser\IMessage
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\IMessage::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newMultipartHelper() : MultipartHelper
diff --git a/tests/MailMimeParser/Message/Helper/PrivacyHelperTest.php b/tests/MailMimeParser/Message/Helper/PrivacyHelperTest.php
index 69fe62a0..ef9b1279 100644
--- a/tests/MailMimeParser/Message/Helper/PrivacyHelperTest.php
+++ b/tests/MailMimeParser/Message/Helper/PrivacyHelperTest.php
@@ -46,12 +46,18 @@ protected function setUp() : void
 
     private function newMockIMimePart() : \ZBateson\MailMimeParser\Message\IMimePart
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\Message\IMimePart::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message\MimePart::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newMockIMessage() : \ZBateson\MailMimeParser\IMessage
     {
-        return $this->getMockForAbstractClass(\ZBateson\MailMimeParser\IMessage::class);
+        return $this
+            ->getMockBuilder(\ZBateson\MailMimeParser\Message::class)
+            ->disableOriginalConstructor()
+            ->getMock();
     }
 
     private function newPrivacyHelper() : PrivacyHelper
diff --git a/tests/MailMimeParser/Parser/Proxy/ParserMimePartProxyTest.php b/tests/MailMimeParser/Parser/Proxy/ParserMimePartProxyTest.php
index 4fc9e81f..fc0bc2d5 100644
--- a/tests/MailMimeParser/Parser/Proxy/ParserMimePartProxyTest.php
+++ b/tests/MailMimeParser/Parser/Proxy/ParserMimePartProxyTest.php
@@ -3,7 +3,7 @@
 namespace ZBateson\MailMimeParser\Parser\Proxy;
 
 use PHPUnit\Framework\TestCase;
-use ZBateson\MailMimeParser\Header\IHeader;
+use ZBateson\MailMimeParser\Header\ParameterHeader;
 
 /**
  * ParserMimePartProxyTest
@@ -263,7 +263,9 @@ public function testGetContentType() : void
             ->method('getHeaderContainer')
             ->willReturn($this->headerContainer);
 
-        $tst = $this->getMockForAbstractClass(IHeader::class);
+        $tst = $this->getMockBuilder(ParameterHeader::class)
+            ->disableOriginalConstructor()
+            ->getMock();
         $this->headerContainer->expects($this->any())
             ->method('get')
             ->with($this->equalTo('Content-Type'))