diff --git a/.travis.yml b/.travis.yml
index a9023fa6..187d4f6f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,7 +7,6 @@ cache:
   - vendor
 
 php:
-  - 5.6
   - 7.0
   - 7.1
   - 7.2
@@ -17,6 +16,7 @@ install:
 
 script:
   - vendor/bin/phpunit
+  - vendor/bin/phpstan analyse -l 6 src
 
 branches:
   only:
diff --git a/composer.json b/composer.json
index d8d11125..86f82c41 100644
--- a/composer.json
+++ b/composer.json
@@ -4,7 +4,7 @@
     "homepage": "https://oauth2.thephpleague.com/",
     "license": "MIT",
     "require": {
-        "php": ">=5.6.0",
+        "php": ">=7.0.0",
         "ext-openssl": "*",
         "league/event": "^2.1",
         "lcobucci/jwt": "^3.1",
@@ -14,7 +14,8 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8.38 || ^5.7.21",
-        "zendframework/zend-diactoros": "^1.0"
+        "zendframework/zend-diactoros": "^1.0",
+        "phpstan/phpstan": "^0.9.2"
     },
     "repositories": [
         {
diff --git a/src/AuthorizationServer.php b/src/AuthorizationServer.php
index 69c16954..876b0083 100644
--- a/src/AuthorizationServer.php
+++ b/src/AuthorizationServer.php
@@ -17,6 +17,7 @@ use League\OAuth2\Server\Repositories\AccessTokenRepositoryInterface;
 use League\OAuth2\Server\Repositories\ClientRepositoryInterface;
 use League\OAuth2\Server\Repositories\ScopeRepositoryInterface;
 use League\OAuth2\Server\RequestTypes\AuthorizationRequest;
+use League\OAuth2\Server\ResponseTypes\AbstractResponseType;
 use League\OAuth2\Server\ResponseTypes\BearerTokenResponse;
 use League\OAuth2\Server\ResponseTypes\ResponseTypeInterface;
 use Psr\Http\Message\ResponseInterface;
@@ -190,7 +191,7 @@ class AuthorizationServer implements EmitterAwareInterface
             if ($tokenResponse instanceof ResponseTypeInterface) {
                 return $tokenResponse->generateHttpResponse($response);
             }
-            
+
         }
 
         throw OAuthServerException::unsupportedGrantType();
@@ -207,7 +208,9 @@ class AuthorizationServer implements EmitterAwareInterface
             $this->responseType = new BearerTokenResponse();
         }
 
-        $this->responseType->setPrivateKey($this->privateKey);
+        if ($this->responseType instanceof AbstractResponseType === true) {
+            $this->responseType->setPrivateKey($this->privateKey);
+        }
         $this->responseType->setEncryptionKey($this->encryptionKey);
 
         return $this->responseType;
diff --git a/src/Entities/AccessTokenEntityInterface.php b/src/Entities/AccessTokenEntityInterface.php
index c297e267..4da7600e 100644
--- a/src/Entities/AccessTokenEntityInterface.php
+++ b/src/Entities/AccessTokenEntityInterface.php
@@ -9,6 +9,7 @@
 
 namespace League\OAuth2\Server\Entities;
 
+use Lcobucci\JWT\Token;
 use League\OAuth2\Server\CryptKey;
 
 interface AccessTokenEntityInterface extends TokenInterface
@@ -18,7 +19,7 @@ interface AccessTokenEntityInterface extends TokenInterface
      *
      * @param CryptKey $privateKey
      *
-     * @return string
+     * @return Token
      */
     public function convertToJWT(CryptKey $privateKey);
 }
diff --git a/src/Entities/RefreshTokenEntityInterface.php b/src/Entities/RefreshTokenEntityInterface.php
index 05e86e00..e4f10400 100644
--- a/src/Entities/RefreshTokenEntityInterface.php
+++ b/src/Entities/RefreshTokenEntityInterface.php
@@ -21,7 +21,7 @@ interface RefreshTokenEntityInterface
     /**
      * Set the token's identifier.
      *
-     * @param $identifier
+     * @param mixed $identifier
      */
     public function setIdentifier($identifier);
 
diff --git a/src/Entities/TokenInterface.php b/src/Entities/TokenInterface.php
index c842b09a..378adbdc 100644
--- a/src/Entities/TokenInterface.php
+++ b/src/Entities/TokenInterface.php
@@ -21,7 +21,7 @@ interface TokenInterface
     /**
      * Set the token's identifier.
      *
-     * @param $identifier
+     * @param mixed $identifier
      */
     public function setIdentifier($identifier);
 
@@ -42,14 +42,14 @@ interface TokenInterface
     /**
      * Set the identifier of the user associated with the token.
      *
-     * @param string|int $identifier The identifier of the user
+     * @param string|int|null $identifier The identifier of the user
      */
     public function setUserIdentifier($identifier);
 
     /**
      * Get the token user's identifier.
      *
-     * @return string|int
+     * @return string|int|null
      */
     public function getUserIdentifier();
 
diff --git a/src/Entities/Traits/AccessTokenTrait.php b/src/Entities/Traits/AccessTokenTrait.php
index 741d6c19..81fc1bfd 100644
--- a/src/Entities/Traits/AccessTokenTrait.php
+++ b/src/Entities/Traits/AccessTokenTrait.php
@@ -12,6 +12,7 @@ namespace League\OAuth2\Server\Entities\Traits;
 use Lcobucci\JWT\Builder;
 use Lcobucci\JWT\Signer\Key;
 use Lcobucci\JWT\Signer\Rsa\Sha256;
+use Lcobucci\JWT\Token;
 use League\OAuth2\Server\CryptKey;
 use League\OAuth2\Server\Entities\ClientEntityInterface;
 use League\OAuth2\Server\Entities\ScopeEntityInterface;
@@ -23,7 +24,7 @@ trait AccessTokenTrait
      *
      * @param CryptKey $privateKey
      *
-     * @return string
+     * @return Token
      */
     public function convertToJWT(CryptKey $privateKey)
     {
diff --git a/src/Entities/Traits/TokenEntityTrait.php b/src/Entities/Traits/TokenEntityTrait.php
index 0b5608cd..c6653cce 100644
--- a/src/Entities/Traits/TokenEntityTrait.php
+++ b/src/Entities/Traits/TokenEntityTrait.php
@@ -25,7 +25,7 @@ trait TokenEntityTrait
     protected $expiryDateTime;
 
     /**
-     * @var string|int
+     * @var string|int|null
      */
     protected $userIdentifier;
 
@@ -77,7 +77,7 @@ trait TokenEntityTrait
     /**
      * Set the identifier of the user associated with the token.
      *
-     * @param string|int $identifier The identifier of the user
+     * @param string|int|null $identifier The identifier of the user
      */
     public function setUserIdentifier($identifier)
     {
@@ -87,7 +87,7 @@ trait TokenEntityTrait
     /**
      * Get the token user's identifier.
      *
-     * @return string|int
+     * @return string|int|null
      */
     public function getUserIdentifier()
     {
diff --git a/src/Exception/OAuthServerException.php b/src/Exception/OAuthServerException.php
index 8d101c4c..756dfd3b 100644
--- a/src/Exception/OAuthServerException.php
+++ b/src/Exception/OAuthServerException.php
@@ -131,7 +131,7 @@ class OAuthServerException extends \Exception
     /**
      * Server error.
      *
-     * @param $hint
+     * @param string $hint
      *
      * @return static
      *
diff --git a/src/Grant/AbstractGrant.php b/src/Grant/AbstractGrant.php
index 25378955..e806ba09 100644
--- a/src/Grant/AbstractGrant.php
+++ b/src/Grant/AbstractGrant.php
@@ -341,7 +341,7 @@ abstract class AbstractGrant implements GrantTypeInterface
      *
      * @param \DateInterval          $accessTokenTTL
      * @param ClientEntityInterface  $client
-     * @param string                 $userIdentifier
+     * @param string|null            $userIdentifier
      * @param ScopeEntityInterface[] $scopes
      *
      * @throws OAuthServerException
diff --git a/src/RequestTypes/AuthorizationRequest.php b/src/RequestTypes/AuthorizationRequest.php
index 41bfb509..ce5a0034 100644
--- a/src/RequestTypes/AuthorizationRequest.php
+++ b/src/RequestTypes/AuthorizationRequest.php
@@ -53,7 +53,7 @@ class AuthorizationRequest
     /**
      * The redirect URI used in the request
      *
-     * @var string
+     * @var string|null
      */
     protected $redirectUri;
 
@@ -159,7 +159,7 @@ class AuthorizationRequest
     }
 
     /**
-     * @return string
+     * @return string|null
      */
     public function getRedirectUri()
     {
@@ -167,7 +167,7 @@ class AuthorizationRequest
     }
 
     /**
-     * @param string $redirectUri
+     * @param string|null $redirectUri
      */
     public function setRedirectUri($redirectUri)
     {
diff --git a/src/ResourceServer.php b/src/ResourceServer.php
index 5e9c13f3..e1f98d6d 100644
--- a/src/ResourceServer.php
+++ b/src/ResourceServer.php
@@ -63,7 +63,9 @@ class ResourceServer
             $this->authorizationValidator = new BearerTokenValidator($this->accessTokenRepository);
         }
 
-        $this->authorizationValidator->setPublicKey($this->publicKey);
+        if ($this->authorizationValidator instanceof BearerTokenValidator === true) {
+            $this->authorizationValidator->setPublicKey($this->publicKey);
+        }
 
         return $this->authorizationValidator;
     }