diff --git a/.travis.yml b/.travis.yml
index a764af4..77c5ea3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -28,17 +28,24 @@ matrix:
         - php: 7.3
     fast_finish: true
     include:
-        - php: 5.5
-          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci" PULI_VERSION=1.0.0-beta9 
-
+        - name: PHPSpec code coverage
+          php: 5.5
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci" PULI_VERSION=1.0.0-beta9 DEPENDENCIES="henrikbjorn/phpspec-code-coverage:^2.0.2"
+        - name: PHPUnit tests
+          php: 7.2
+          env: TEST_COMMAND="./vendor/bin/phpunit" DEPENDENCIES="phpunit/phpunit:^7.5 nyholm/psr7:^1.0 kriswallsmith/buzz:^1.0@beta php-http/curl-client:^1.0 php-http/message"
+        - name: PHPUnit test with nothing installed
+          php: 7.2
+          env: TEST_COMMAND="./vendor/bin/phpunit --group=NothingInstalled" DEPENDENCIES="phpunit/phpunit:^7.5"
 
 before_install:
     - if [[ $COVERAGE != true ]]; then phpenv config-rm xdebug.ini || true; fi
+    - if ! [ -z "$DEPENDENCIES" ]; then composer require --no-update ${DEPENDENCIES}; fi;
 
 install:
     - wget https://github.com/puli/cli/releases/download/1.0.0-beta10/puli.phar && chmod +x puli.phar
     - composer require puli/composer-plugin:${PULI_VERSION} --no-update
-    - travis_retry composer update ${COMPOSER_FLAGS} --prefer-dist --no-interaction
+    - composer update ${COMPOSER_FLAGS} --prefer-dist --no-interaction
 
 script:
     - $TEST_COMMAND
diff --git a/composer.json b/composer.json
index df1e31a..e6b6414 100644
--- a/composer.json
+++ b/composer.json
@@ -17,8 +17,7 @@
         "php-http/httplug": "^1.0 || ^2.0",
         "php-http/message-factory": "^1.0",
         "puli/composer-plugin": "1.0.0-beta10",
-        "phpspec/phpspec": "^2.4",
-        "henrikbjorn/phpspec-code-coverage" : "^2.0.2"
+        "phpspec/phpspec": "^2.4"
     },
     "suggest": {
         "puli/composer-plugin": "Sets up Puli which is recommended for Discovery to work. Check http://docs.php-http.org/en/latest/discovery.html for more details.",
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
new file mode 100644
index 0000000..72a6c47
--- /dev/null
+++ b/phpunit.xml.dist
@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<phpunit bootstrap="./vendor/autoload.php"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true">
+
+    <testsuites>
+        <testsuite name="HTTPlug unit tests">
+            <directory suffix="Test.php">./tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <groups>
+        <exclude>
+            <group>NothingInstalled</group>
+        </exclude>
+    </groups>
+</phpunit>
diff --git a/src/Exception/NoCandidateFoundException.php b/src/Exception/NoCandidateFoundException.php
index 240b568..e53fb79 100644
--- a/src/Exception/NoCandidateFoundException.php
+++ b/src/Exception/NoCandidateFoundException.php
@@ -27,9 +27,22 @@ function ($a) {
         $message = sprintf(
             'No valid candidate found using strategy "%s". We tested the following candidates: %s.',
             $strategy,
-            implode(', ', $classes)
+            implode(', ', array_map([$this, 'stringify'], $classes))
         );
 
         parent::__construct($message);
     }
+
+    private function stringify($mixed)
+    {
+        if (is_string($mixed)) {
+            return $mixed;
+        }
+
+        if (is_array($mixed) && 2 === count($mixed)) {
+            return sprintf('%s::%s', $this->stringify($mixed[0]), $mixed[1]);
+        }
+
+        return is_object($mixed) ? get_class($mixed) : gettype($mixed);
+    }
 }
diff --git a/src/Psr17FactoryDiscovery.php b/src/Psr17FactoryDiscovery.php
index 1aa833a..7c70fb4 100644
--- a/src/Psr17FactoryDiscovery.php
+++ b/src/Psr17FactoryDiscovery.php
@@ -20,7 +20,7 @@ final class Psr17FactoryDiscovery extends ClassDiscovery
     private static function createException($type, Exception $e)
     {
         return new \Http\Discovery\Exception\NotFoundException(
-            'No psr-17 '.$type.' found. Install a package from this list: https://packagist.org/providers/psr/http-factory-implementation',
+            'No PSR-17 '.$type.' found. Install a package from this list: https://packagist.org/providers/psr/http-factory-implementation',
             0,
             $e
         );
diff --git a/src/Strategy/CommonClassesStrategy.php b/src/Strategy/CommonClassesStrategy.php
index c2b7e09..ab7e4c0 100644
--- a/src/Strategy/CommonClassesStrategy.php
+++ b/src/Strategy/CommonClassesStrategy.php
@@ -81,7 +81,12 @@ final class CommonClassesStrategy implements DiscoveryStrategy
                 'condition' => [\Buzz\Client\FileGetContents::class, \Buzz\Message\ResponseBuilder::class],
             ],
         ],
-        Psr18Client::class => [],
+        Psr18Client::class => [
+            [
+                'class' => [self::class, 'buzzInstantiate'],
+                'condition' => [\Buzz\Client\FileGetContents::class, \Buzz\Message\ResponseBuilder::class],
+            ],
+        ],
     ];
 
     /**
diff --git a/tests/Exception/InitializeExceptionTest.php b/tests/Exception/InitializeExceptionTest.php
new file mode 100644
index 0000000..a9cdc56
--- /dev/null
+++ b/tests/Exception/InitializeExceptionTest.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace tests\Http\Discovery\Exception;
+
+use Http\Discovery\Exception as DiscoveryException;
+use PHPUnit\Framework\TestCase;
+
+class InitializeExceptionTest extends TestCase
+{
+    public function testInitialize()
+    {
+        $e[] = new DiscoveryException\ClassInstantiationFailedException();
+        $e[] = new DiscoveryException\NotFoundException();
+        $e[] = new DiscoveryException\PuliUnavailableException();
+        $e[] = new DiscoveryException\StrategyUnavailableException();
+        $e[] = new DiscoveryException\NoCandidateFoundException('CommonClasses', []);
+        $e[] = DiscoveryException\DiscoveryFailedException::create($e);
+
+        foreach ($e as $exception) {
+            $this->assertInstanceOf(\Http\Discovery\Exception::class, $exception);
+        }
+    }
+}
diff --git a/tests/Exception/NoCandidateFoundExceptionTest.php b/tests/Exception/NoCandidateFoundExceptionTest.php
new file mode 100644
index 0000000..a6c2a6b
--- /dev/null
+++ b/tests/Exception/NoCandidateFoundExceptionTest.php
@@ -0,0 +1,67 @@
+<?php
+
+namespace tests\Http\Discovery\Exception;
+
+use Http\Discovery\Exception as DiscoveryException;
+use PHPUnit\Framework\TestCase;
+
+class NoCandidateFoundExceptionTest extends TestCase
+{
+    public function testInitialize()
+    {
+        $candidates = [
+            ['class' => 'Foo', 'condition' => true],
+            ['class' => 'Bar', 'condition' => false],
+        ];
+        $exception = new DiscoveryException\NoCandidateFoundException('Foobar', $candidates);
+        $this->assertInstanceOf(\Http\Discovery\Exception::class, $exception);
+        $this->assertStringStartsWith('No valid candidate found using strategy "Foobar".', $exception->getMessage());
+    }
+
+    public function testInitializeWithArrayCallable()
+    {
+        $candidates = [
+            ['class' => ['AnyClass', 'anyFunction'], 'condition' => true],
+        ];
+
+        $exception = new DiscoveryException\NoCandidateFoundException('Foobar', $candidates);
+        $this->assertInstanceOf(\Http\Discovery\Exception::class, $exception);
+    }
+
+    public function testInitializeWithObjectCallable()
+    {
+        $obj = new \stdClass();
+        $candidates = [
+            ['class' => [$obj, 'anyFunction'], 'condition' => true],
+        ];
+
+        $exception = new DiscoveryException\NoCandidateFoundException('Foobar', $candidates);
+        $this->assertInstanceOf(\Http\Discovery\Exception::class, $exception);
+    }
+
+    public function testInitializeWithClosure()
+    {
+        $obj = \Closure::fromCallable(function () {
+            $x = 2;
+        });
+        $candidates = [
+            ['class' => $obj, 'condition' => true],
+        ];
+
+        $exception = new DiscoveryException\NoCandidateFoundException('Foobar', $candidates);
+        $this->assertInstanceOf(\Http\Discovery\Exception::class, $exception);
+    }
+
+    public function testInitializeWithAnonymousFunction()
+    {
+        $func = function () {
+            $x = 2;
+        };
+        $candidates = [
+            ['class' => $func, 'condition' => true],
+        ];
+
+        $exception = new DiscoveryException\NoCandidateFoundException('Foobar', $candidates);
+        $this->assertInstanceOf(\Http\Discovery\Exception::class, $exception);
+    }
+}
diff --git a/tests/Psr17FactoryDiscoveryTest.php b/tests/Psr17FactoryDiscoveryTest.php
new file mode 100644
index 0000000..a9d8797
--- /dev/null
+++ b/tests/Psr17FactoryDiscoveryTest.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace tests\Http\Discovery;
+
+use Http\Discovery\Exception\NotFoundException;
+use Http\Discovery\Psr17FactoryDiscovery;
+use PHPUnit\Framework\TestCase;
+use Psr\Http\Message\RequestFactoryInterface;
+use Psr\Http\Message\ResponseFactoryInterface;
+use Psr\Http\Message\ServerRequestFactoryInterface;
+use Psr\Http\Message\StreamFactoryInterface;
+use Psr\Http\Message\UploadedFileFactoryInterface;
+use Psr\Http\Message\UriFactoryInterface;
+
+class Psr17FactoryDiscoveryTest extends TestCase
+{
+    /**
+     * @dataProvider getFactories
+     */
+    public function testFind($method, $interface)
+    {
+        $callable = [Psr17FactoryDiscovery::class, $method];
+        $client = $callable();
+        $this->assertInstanceOf($interface, $client);
+    }
+
+    /**
+     * @group NothingInstalled
+     * @dataProvider getFactories
+     */
+    public function testNotFound($method)
+    {
+        $callable = [Psr17FactoryDiscovery::class, $method];
+        $this->expectException(NotFoundException::class);
+        $callable();
+    }
+
+    public function getFactories()
+    {
+        yield ['findRequestFactory', RequestFactoryInterface::class];
+        yield ['findResponseFactory', ResponseFactoryInterface::class];
+        yield ['findServerRequestFactory', ServerRequestFactoryInterface::class];
+        yield ['findStreamFactory', StreamFactoryInterface::class];
+        yield ['findUploadedFileFactory', UploadedFileFactoryInterface::class];
+        yield ['findUrlFactory', UriFactoryInterface::class];
+    }
+}
diff --git a/tests/Psr18ClientDiscoveryTest.php b/tests/Psr18ClientDiscoveryTest.php
new file mode 100644
index 0000000..30657b8
--- /dev/null
+++ b/tests/Psr18ClientDiscoveryTest.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace tests\Http\Discovery;
+
+use Http\Discovery\Exception\NotFoundException;
+use Http\Discovery\Psr18ClientDiscovery;
+use PHPUnit\Framework\TestCase;
+use Psr\Http\Client\ClientInterface;
+
+class Psr18ClientDiscoveryTest extends TestCase
+{
+    public function testFind()
+    {
+        $client = Psr18ClientDiscovery::find();
+        $this->assertInstanceOf(ClientInterface::class, $client);
+    }
+
+    /**
+     * @group NothingInstalled
+     */
+    public function testNotFound()
+    {
+        $this->expectException(NotFoundException::class);
+        $client = Psr18ClientDiscovery::find();
+    }
+}