diff --git a/src/Application.php b/src/Application.php
index 3d9bcdb6..11ef2b29 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -2,9 +2,9 @@
 
 namespace PhpSchool\PhpWorkshop;
 
-use Assert\Assertion;
 use DI\ContainerBuilder;
 use PhpSchool\PhpWorkshop\Check\CheckRepository;
+use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
 use PhpSchool\PhpWorkshop\Exception\MissingArgumentException;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\Factory\ResultRendererFactory;
@@ -70,8 +70,9 @@ final class Application
      */
     public function __construct(string $workshopTitle, string $diConfigFile)
     {
-        Assertion::string($workshopTitle);
-        Assertion::file($diConfigFile);
+        if (!\is_file($diConfigFile)) {
+            throw new InvalidArgumentException(\sprintf('File "%s" was expected to exist.', $diConfigFile));
+        }
 
         $this->workshopTitle = $workshopTitle;
         $this->diConfigFile = $diConfigFile;
@@ -105,8 +106,13 @@ public function addExercise(string $exercise): void
      */
     public function addResult(string $resultClass, string $resultRendererClass): void
     {
-        Assertion::classExists($resultClass);
-        Assertion::classExists($resultRendererClass);
+        if (!\class_exists($resultClass)) {
+            throw new InvalidArgumentException(\sprintf('Class "%s" does not exist', $resultClass));
+        }
+
+        if (!\class_exists($resultRendererClass)) {
+            throw new InvalidArgumentException(\sprintf('Class "%s" does not exist', $resultRendererClass));
+        }
 
         $this->results[] = [
             'resultClass' => $resultClass,
@@ -122,7 +128,6 @@ public function addResult(string $resultClass, string $resultRendererClass): voi
      */
     public function setLogo(string $logo): void
     {
-        Assertion::string($logo);
         $this->logo = $logo;
     }
 
@@ -134,7 +139,6 @@ public function setLogo(string $logo): void
      */
     public function setFgColour(string $colour): void
     {
-        Assertion::string($colour);
         $this->fgColour = $colour;
     }
 
@@ -146,7 +150,6 @@ public function setFgColour(string $colour): void
      */
     public function setBgColour(string $colour): void
     {
-        Assertion::string($colour);
         $this->bgColour = $colour;
     }
 
diff --git a/src/CodeInsertion.php b/src/CodeInsertion.php
index b1866a7e..d7cd8fa1 100644
--- a/src/CodeInsertion.php
+++ b/src/CodeInsertion.php
@@ -2,7 +2,7 @@
 
 namespace PhpSchool\PhpWorkshop;
 
-use Assert\Assertion;
+use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
 
 /**
  * This class is a simple DTO to represent a code insertion which should
@@ -49,7 +49,11 @@ class CodeInsertion
      */
     public function __construct(string $type, string $code)
     {
-        Assertion::inArray($type, $this->types);
+        if (!in_array($type, $this->types, true)) {
+            throw new InvalidArgumentException(
+                sprintf('Value "%s" is not an element of the valid values: %s', $type, implode(', ', $this->types))
+            );
+        }
 
         $this->type = $type;
         $this->code = $code;
diff --git a/src/Event/CliExecuteEvent.php b/src/Event/CliExecuteEvent.php
index 43d520b7..ea2e5361 100644
--- a/src/Event/CliExecuteEvent.php
+++ b/src/Event/CliExecuteEvent.php
@@ -2,7 +2,6 @@
 
 namespace PhpSchool\PhpWorkshop\Event;
 
-use Assert\Assertion;
 use PhpSchool\PhpWorkshop\Utils\ArrayObject;
 
 /**
diff --git a/test/CodeInsertionTest.php b/test/CodeInsertionTest.php
index b385ca71..2baf2c88 100644
--- a/test/CodeInsertionTest.php
+++ b/test/CodeInsertionTest.php
@@ -2,8 +2,8 @@
 
 namespace PhpSchool\PhpWorkshopTest;
 
-use Assert\InvalidArgumentException;
 use PhpSchool\PhpWorkshop\CodeInsertion;
+use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
 use PHPUnit\Framework\TestCase;
 
 class CodeInsertionTest extends TestCase