diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1cbaa815e0..c9c3548e4e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ Updates should follow the [Keep a CHANGELOG](https://keepachangelog.com/) princi
 
 ## [Unreleased][unreleased]
 
+### Added
+
+ - Added check to ensure Markdown input is valid UTF-8 (#401, #405)
+
 ## [1.2.2] - 2019-01-15
 
 This release contains the same changes as 1.1.3:
diff --git a/src/Converter.php b/src/Converter.php
index 500f0b3e81..38d2abc4d0 100644
--- a/src/Converter.php
+++ b/src/Converter.php
@@ -50,6 +50,8 @@ public function __construct(DocParserInterface $docParser, ElementRendererInterf
      *
      * @param string $commonMark
      *
+     * @throws \RuntimeException
+     *
      * @return string
      *
      * @api
@@ -68,6 +70,8 @@ public function convertToHtml(string $commonMark): string
      *
      * @param string $commonMark
      *
+     * @throws \RuntimeException
+     *
      * @return string
      */
     public function __invoke(string $commonMark): string
diff --git a/src/DocParser.php b/src/DocParser.php
index 496eed80bb..dfc87d5327 100644
--- a/src/DocParser.php
+++ b/src/DocParser.php
@@ -20,6 +20,7 @@
 use League\CommonMark\Block\Element\Paragraph;
 use League\CommonMark\Block\Element\StringContainerInterface;
 use League\CommonMark\Event\DocumentParsedEvent;
+use League\CommonMark\Exception\UnexpectedEncodingException;
 
 final class DocParser implements DocParserInterface
 {
@@ -71,6 +72,8 @@ private function preProcessInput(string $input): array
     /**
      * @param string $input
      *
+     * @throws \RuntimeException
+     *
      * @return Document
      */
     public function parse(string $input): Document
@@ -78,6 +81,7 @@ public function parse(string $input): Document
         $document = new Document();
         $context = new Context($document, $this->environment);
 
+        $this->assertValidUTF8($input);
         $lines = $this->preProcessInput($input);
         foreach ($lines as $line) {
             $context->setNextLine($line);
@@ -248,4 +252,11 @@ private function setAndPropagateLastLineBlank(ContextInterface $context, Cursor
             $container = $container->parent();
         }
     }
+
+    private function assertValidUTF8(string $input)
+    {
+        if (!\mb_check_encoding($input, 'UTF-8')) {
+            throw new UnexpectedEncodingException('Unexpected encoding - UTF-8 or ASCII was expected');
+        }
+    }
 }
diff --git a/src/DocParserInterface.php b/src/DocParserInterface.php
index f43c93093e..23725ad476 100644
--- a/src/DocParserInterface.php
+++ b/src/DocParserInterface.php
@@ -18,6 +18,8 @@ interface DocParserInterface
     /**
      * @param string $input
      *
+     * @throws \RuntimeException
+     *
      * @return Document
      */
     public function parse(string $input): Document;
diff --git a/src/Exception/UnexpectedEncodingException.php b/src/Exception/UnexpectedEncodingException.php
new file mode 100644
index 0000000000..ada054bc90
--- /dev/null
+++ b/src/Exception/UnexpectedEncodingException.php
@@ -0,0 +1,16 @@
+<?php
+
+/*
+ * This file is part of the league/commonmark package.
+ *
+ * (c) Colin O'Dell <colinodell@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace League\CommonMark\Exception;
+
+final class UnexpectedEncodingException extends \RuntimeException
+{
+}
diff --git a/tests/unit/CommonMarkConverterTest.php b/tests/unit/CommonMarkConverterTest.php
index 16af95374a..7f24f44df1 100644
--- a/tests/unit/CommonMarkConverterTest.php
+++ b/tests/unit/CommonMarkConverterTest.php
@@ -60,4 +60,13 @@ public function testEnvironmentAndConfigConstructor()
 
         $this->assertSame($mockEnvironment, $environment);
     }
+
+    /**
+     * @expectedException \League\CommonMark\Exception\UnexpectedEncodingException
+     */
+    public function testConvertingInvalidUTF8()
+    {
+        $converter = new CommonMarkConverter();
+        $converter->convertToHtml("\x09\xca\xca");
+    }
 }
diff --git a/tests/unit/DocParserTest.php b/tests/unit/DocParserTest.php
new file mode 100644
index 0000000000..5e5319fe3a
--- /dev/null
+++ b/tests/unit/DocParserTest.php
@@ -0,0 +1,30 @@
+<?php
+
+/*
+ * This file is part of the league/commonmark package.
+ *
+ * (c) Colin O'Dell <colinodell@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace League\CommonMark\Tests\Unit;
+
+use League\CommonMark\DocParser;
+use League\CommonMark\Environment;
+use PHPUnit\Framework\TestCase;
+
+class DocParserTest extends TestCase
+{
+    /**
+     * @expectedException \League\CommonMark\Exception\UnexpectedEncodingException
+     */
+    public function testParsingWithInvalidUTF8()
+    {
+        $environment = Environment::createCommonMarkEnvironment();
+        $docParser = new DocParser($environment);
+
+        $docParser->parse("\x09\xca\xca");
+    }
+}